[Fix, Test] (MG_Backend, MG_Impl, MG_State): advertise the uniform-block and stencil-texturing strings a 4.0 context hides behind, forward DEPTH_STENCIL_TEXTURE_MODE to both backends, and stop a reserved transform feedback name passing for an object

MobileGL/MG_Backend/DirectGLES/BackendObject_DirectGLES.cpp

@@ -940,6 +940,17 @@ namespace MobileGL::MG_Backend::DirectGLES {
             // picks a whole different shader for draw_buffers without
             // explicit_attrib_location. DirectVulkan advertises both.
             E_GL_ARB_explicit_attrib_location, E_GL_ARB_texture_multisample, E_GL_ARB_shader_image_size,
+            // Core since GL 3.1 and implemented for every version advertised here. The string
+            // matters because applications gate the ENTRY POINTS on it rather than on the
+            // version: a caller that finds the extension missing never resolves
+            // glGetUniformBlockIndex / glUniformBlockBinding, and one that then uses uniform
+            // blocks anyway calls through a null pointer.
+            E_GL_ARB_uniform_buffer_object,
+            // Sampling the stencil aspect through DEPTH_STENCIL_TEXTURE_MODE. Core from 4.3,
+            // so on a 4.0 context the string is the only way to reach it. The host ES driver
+            // has had the same texture parameter since ES 3.1, which every device MobileGL
+            // runs on provides.
+            E_GL_ARB_stencil_texturing,
             // Advertised with GL_NUM_PROGRAM_BINARY_FORMATS = 0, which the
             // extension explicitly permits. It is also the only thing that
             // exposes glProgramParameteri before GL 4.1.

MobileGL/MG_Backend/DirectGLES/Managers.cpp

@@ -3315,6 +3315,29 @@ namespace MobileGL::MG_Backend::DirectGLES {
                     MGLOG_D("%s(%s:%d) ES error %s", func, file, line, MG_Util::ConvertGLEnumToString(err).c_str());
                 });
             }
+
+            // GL_DEPTH_STENCIL_TEXTURE_MODE (GL_ARB_stencil_texturing / ES 3.1 core): which aspect
+            // of a packed depth/stencil image a sampler reads. Until this was forwarded the
+            // frontend kept the mode as a pure shadow - glGetTexParameter answered it, sampling
+            // ignored it - so a usampler2D bound to a D24S8 texture in STENCIL_INDEX mode read the
+            // depth aspect. Texture state rather than sampler state, so multisample targets take
+            // it too (ES 3.1 8.10 lists it among the three pnames they accept). It is only sent
+            // when it has moved, which for the overwhelming majority of textures is never.
+            const Bool supportsStencilTextureMode =
+                g_GLESCapabilities.GLESVersion.Major > 3 ||
+                (g_GLESCapabilities.GLESVersion.Major == 3 && g_GLESCapabilities.GLESVersion.Minor >= 1);
+            if (supportsStencilTextureMode) {
+                const GLenum depthStencilTextureMode = stateTextureObject->GetDepthStencilTextureMode();
+                if (m_cacheDepthStencilTextureMode != depthStencilTextureMode) {
+                    g_GLESFuncs.glTexParameteri(target, GL_DEPTH_STENCIL_TEXTURE_MODE,
+                                                static_cast<GLint>(depthStencilTextureMode));
+                    m_cacheDepthStencilTextureMode = depthStencilTextureMode;
+                    DebugImpl::ErrorLopper::Loop([file = __FILE__, line = __LINE__, func = __func__](GLenum err) {
+                        MGLOG_D("%s(%s:%d) ES error %s", func, file, line,
+                                MG_Util::ConvertGLEnumToString(err).c_str());
+                    });
+                }
+            }
         }
 
         void ActivateTextureUnit(Uint unit) {

MobileGL/MG_Backend/DirectGLES/Managers.h

@@ -699,6 +699,12 @@ namespace MobileGL::MG_Backend::DirectGLES {
             FloatVec4 m_cacheBorderColor = {0.0f, 0.0f, 0.0f, 0.0f};
             Vec4<TextureSwizzleParam> m_cacheSwizzleParams = {TextureSwizzleParam::Red, TextureSwizzleParam::Green,
                                                               TextureSwizzleParam::Blue, TextureSwizzleParam::Alpha};
+            // GL_DEPTH_STENCIL_TEXTURE_MODE. GL_DEPTH_COMPONENT is the GL and ES default, so a
+            // texture that never asks for the stencil aspect never emits the call. The
+            // depth/stencil readback and replicate-blit emulations also write this parameter
+            // raw, but only ever on their own scratch textures (never on an application
+            // texture), so they cannot desynchronise this cache.
+            GLenum m_cacheDepthStencilTextureMode = GL_DEPTH_COMPONENT;
             Uint16 m_syncedSamplerVersion = 0;
             Uint16 m_syncedTextureParamsVersion = 0;
         };

MobileGL/MG_Backend/DirectVulkan/BackendObject_DirectVulkan.cpp

@@ -517,6 +517,15 @@ namespace MobileGL::MG_Backend::DirectVulkan {
             E_GL_ARB_gpu_shader_int64, E_GL_KHR_debug, E_GL_ARB_gpu_shader5, E_GL_ARB_multi_bind,
             E_GL_ARB_shading_language_420pack, E_GL_ARB_vertex_attrib_binding, E_GL_ARB_shader_image_size,
             E_GL_ARB_explicit_attrib_location,
+            // Core since GL 3.1 and implemented for every version advertised here. The string
+            // matters because applications gate the ENTRY POINTS on it rather than on the
+            // version: a caller that finds the extension missing never resolves
+            // glGetUniformBlockIndex / glUniformBlockBinding, and one that then uses uniform
+            // blocks anyway calls through a null pointer.
+            E_GL_ARB_uniform_buffer_object,
+            // Sampling the stencil aspect through DEPTH_STENCIL_TEXTURE_MODE. Core from 4.3,
+            // so on a 4.0 context the string is the only way to reach it.
+            E_GL_ARB_stencil_texturing,
             // Advertised with GL_NUM_PROGRAM_BINARY_FORMATS = 0, which the
             // extension explicitly permits. It is also the only thing that
             // exposes glProgramParameteri before GL 4.1.

MobileGL/MG_Backend/DirectVulkan/Renderer/VkTextureManager.cpp

@@ -950,7 +950,8 @@ namespace MobileGL::MG_Backend::DirectVulkan {
 
         const TextureFormatInfo formatInfo = ResolveTextureFormatInfo(texture.GetFormat());
         const VkComponentMapping sampledComponents = ResolveSampledViewComponents(texture, formatInfo);
-        const VkImageAspectFlags sampledAspect = ResolveSampledImageViewAspectMask(resource->aspect);
+        const VkImageAspectFlags sampledAspect =
+            ResolveSampledImageViewAspectMask(resource->aspect, texture.GetDepthStencilTextureMode());
         perMipSampledView = CreateImageView(resource->image, resource->format, sampledAspect, resource->viewType,
                                             mipLevel, 1, 0, resource->arrayLayers, &sampledComponents);
         if (perMipSampledView == VK_NULL_HANDLE) {
@@ -2238,7 +2239,8 @@ namespace MobileGL::MG_Backend::DirectVulkan {
         if (resource.fullView == VK_NULL_HANDLE) {
             return false;
         }
-        const VkImageAspectFlags sampledAspect = ResolveSampledImageViewAspectMask(resource.aspect);
+        const VkImageAspectFlags sampledAspect =
+            ResolveSampledImageViewAspectMask(resource.aspect, texture.GetDepthStencilTextureMode());
         resource.sampledView = CreateImageView(resource.image, resource.format, sampledAspect, resource.viewType,
                                                baseMipLevel, levelCount, 0, resource.arrayLayers, &sampledComponents);
         if (resource.sampledView == VK_NULL_HANDLE) {
@@ -2860,10 +2862,19 @@ namespace MobileGL::MG_Backend::DirectVulkan {
         }
     }
 
-    VkImageAspectFlags VkTextureManager::ResolveSampledImageViewAspectMask(VkImageAspectFlags imageAspect) {
+    VkImageAspectFlags VkTextureManager::ResolveSampledImageViewAspectMask(VkImageAspectFlags imageAspect,
+                                                                           GLenum depthStencilTextureMode) {
         if ((imageAspect & VK_IMAGE_ASPECT_COLOR_BIT) != 0) {
             return VK_IMAGE_ASPECT_COLOR_BIT;
         }
+        // A sampled view of a combined depth/stencil image may name exactly one aspect
+        // (VUID-VkDescriptorImageInfo-imageView-01976), and GL_DEPTH_STENCIL_TEXTURE_MODE is
+        // what picks it - the whole content of GL_ARB_stencil_texturing. Depth stays the
+        // default, so nothing that never sets the mode changes shape. The texture's params
+        // version moves with the mode, which is what makes the cached views be rebuilt.
+        if (depthStencilTextureMode == GL_STENCIL_INDEX && (imageAspect & VK_IMAGE_ASPECT_STENCIL_BIT) != 0) {
+            return VK_IMAGE_ASPECT_STENCIL_BIT;
+        }
         if ((imageAspect & VK_IMAGE_ASPECT_DEPTH_BIT) != 0) {
             return VK_IMAGE_ASPECT_DEPTH_BIT;
         }

MobileGL/MG_Backend/DirectVulkan/Renderer/VkTextureManager.h

@@ -379,7 +379,11 @@ public:
     // true - a false positive merely ends the render pass, a false negative would skip a barrier.
     Bool NeedsStorageImagePreparation(MG_State::GLState::ITextureObject& texture) const;
 
-    static VkImageAspectFlags ResolveSampledImageViewAspectMask(VkImageAspectFlags imageAspect);
+    // `depthStencilTextureMode` is the texture's GL_DEPTH_STENCIL_TEXTURE_MODE; it only decides
+    // anything for an image that carries both aspects. Defaulted so the call sites that have no
+    // texture in hand keep the depth-aspect answer they have always given.
+    static VkImageAspectFlags ResolveSampledImageViewAspectMask(VkImageAspectFlags imageAspect,
+                                                                GLenum depthStencilTextureMode = GL_DEPTH_COMPONENT);
     static VkFormat ResolveSampledImageViewFormat(VkFormat imageFormat, SamplerNumericDomain numericDomain);
     static Bool AreSampledImageViewFormatsCompatible(VkFormat imageFormat, VkFormat viewFormat);
     static Bool AreStorageImageViewFormatsCompatible(VkFormat imageFormat, VkFormat viewFormat);

MobileGL/MG_Impl/GLImpl/Drawing/GL_Drawing.cpp

@@ -1259,7 +1259,15 @@ namespace MobileGL::MG_Impl::GLImpl {
                 MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", functionName, "instancecount must be non-negative."));
             return;
         }
-        if (!MG_State::pGLContext->ValidateTransformFeedbackName(id)) {
+        // "id is not the name of a transform feedback object" has to mean the same thing here
+        // as it does to glIsTransformFeedback, and the two predicates are not interchangeable:
+        // a name glGenTransformFeedbacks handed out is only reserved until it is first bound,
+        // and only the bind turns it into an object (GL 4.6 core 13.2.1). ValidateTransformFeedbackName
+        // answers the reservation question - the right one for glBindTransformFeedback, which is
+        // what turns a reserved name into an object - so using it here let a generated-but-unbound
+        // name through to the completed-span check below and raised INVALID_OPERATION where the
+        // spec asks for INVALID_VALUE. Name 0 is the default object and always drawable.
+        if (id != 0 && !MG_State::pGLContext->IsTransformFeedbackObject(id)) {
             MG_State::pGLContext->RecordError(
                 ErrorCode::InvalidValue,
                 MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", functionName,

MobileGL/MG_State/GLState/TextureState/TextureObject.h

@@ -117,7 +117,15 @@ namespace MobileGL::MG_State::GLState {
         void SetFixedSampleLocations(Bool fixedSampleLocations) override;
         Uint64 GetLifetimeId() const override;
         GLenum GetDepthStencilTextureMode() const override { return m_depthStencilTextureMode; }
-        void SetDepthStencilTextureMode(GLenum mode) override { m_depthStencilTextureMode = mode; }
+        // Bumps the params version like every other backend-visible texture parameter: the mode
+        // decides which ASPECT of a packed depth/stencil image a sampler reads, which DirectGLES
+        // forwards as a texture parameter and DirectVulkan bakes into the sampled image view. A
+        // silent write here would leave both backends showing the aspect they last built.
+        void SetDepthStencilTextureMode(GLenum mode) override {
+            if (m_depthStencilTextureMode == mode) return;
+            m_depthStencilTextureMode = mode;
+            ++m_textureParamsVersion;
+        }
 
     protected:
         static Uint64 AllocateLifetimeId();

MobileGL/MG_Test/SanityTest.cpp

@@ -198,6 +198,38 @@ TEST(DirectGLESSanity, AdvertisesDepthTextureForGlmarkShadowScenes) {
     EXPECT_NE(std::find(extensions.begin(), extensions.end(), MobileGL::E_GL_ARB_depth_texture), extensions.end());
 }
 
+// Two strings that name capabilities MobileGL has always had, and that were missing from the
+// advertised list for as long as it existed.
+//
+// GL_ARB_uniform_buffer_object is the one with teeth: applications gate the ENTRY POINTS on the
+// string rather than on the context version. KHR-GL4x.transform_feedback.draw_xfb_instanced_test
+// resolves glGetUniformBlockIndex / glUniformBlockBinding only inside `if (is_arb_ubo)`, then
+// calls them unconditionally because the context claims >= 4.2 - so a missing string turned into
+// a call through a null pointer and took the whole process down with SIGSEGV. Withdrawing it
+// again would restore that crash on both backends.
+//
+// GL_ARB_stencil_texturing is what makes DEPTH_STENCIL_TEXTURE_MODE = GL_STENCIL_INDEX reachable
+// at all before GL 4.3, which is the whole of KHR-GL3x.packed_depth_stencil.stencil_texturing.
+TEST(DirectGLESSanity, AdvertisesUniformBufferObjectAndStencilTexturing) {
+    MobileGL::MG_Backend::DirectGLES::BackendObject_DirectGLES backend;
+    const auto& extensions = backend.GetRendererInfo().RendererGLInfo.Extensions;
+
+    EXPECT_NE(std::find(extensions.begin(), extensions.end(), MobileGL::E_GL_ARB_uniform_buffer_object),
+              extensions.end());
+    EXPECT_NE(std::find(extensions.begin(), extensions.end(), MobileGL::E_GL_ARB_stencil_texturing),
+              extensions.end());
+}
+
+TEST(DirectVulkanSanity, AdvertisesUniformBufferObjectAndStencilTexturing) {
+    MobileGL::MG_Backend::DirectVulkan::BackendObject_DirectVulkan backend;
+    const auto& extensions = backend.GetRendererInfo().RendererGLInfo.Extensions;
+
+    EXPECT_NE(std::find(extensions.begin(), extensions.end(), MobileGL::E_GL_ARB_uniform_buffer_object),
+              extensions.end());
+    EXPECT_NE(std::find(extensions.begin(), extensions.end(), MobileGL::E_GL_ARB_stencil_texturing),
+              extensions.end());
+}
+
 // Voxy only ever needed the extensions, which stay advertised whatever the version is; the version
 // assertion just pins what the backend really reports, now that V_OpenGL40 is in the list.
 TEST(DirectGLESSanity, AdvertisesVoxyRequiredRenderingExtensions) {
@@ -435,6 +467,31 @@ TEST(DirectVulkanSanity, AdvertisesTextureStorageForDirectStateAccess) {
     EXPECT_NE(std::find(extensions.begin(), extensions.end(), MobileGL::E_GL_ARB_texture_storage), extensions.end());
 }
 
+// A sampled view of a combined depth/stencil image may name exactly one aspect, and
+// GL_DEPTH_STENCIL_TEXTURE_MODE picks which - the whole of GL_ARB_stencil_texturing on this
+// backend. Depth remains the answer for everything that does not ask for stencil, including
+// depth-only images asked for the stencil aspect they do not have.
+TEST(DirectVulkanSanity, SampledViewAspectFollowsDepthStencilTextureMode) {
+    using MobileGL::MG_Backend::DirectVulkan::VkTextureManager;
+    constexpr VkImageAspectFlags kPacked = VK_IMAGE_ASPECT_DEPTH_BIT | VK_IMAGE_ASPECT_STENCIL_BIT;
+
+    EXPECT_EQ(VkTextureManager::ResolveSampledImageViewAspectMask(kPacked, GL_DEPTH_COMPONENT),
+              static_cast<VkImageAspectFlags>(VK_IMAGE_ASPECT_DEPTH_BIT));
+    EXPECT_EQ(VkTextureManager::ResolveSampledImageViewAspectMask(kPacked, GL_STENCIL_INDEX),
+              static_cast<VkImageAspectFlags>(VK_IMAGE_ASPECT_STENCIL_BIT));
+    // The default argument is the pre-existing behaviour, for the call sites with no texture.
+    EXPECT_EQ(VkTextureManager::ResolveSampledImageViewAspectMask(kPacked),
+              static_cast<VkImageAspectFlags>(VK_IMAGE_ASPECT_DEPTH_BIT));
+
+    // Single-aspect images ignore the mode: there is only one aspect to name.
+    EXPECT_EQ(VkTextureManager::ResolveSampledImageViewAspectMask(VK_IMAGE_ASPECT_DEPTH_BIT, GL_STENCIL_INDEX),
+              static_cast<VkImageAspectFlags>(VK_IMAGE_ASPECT_DEPTH_BIT));
+    EXPECT_EQ(VkTextureManager::ResolveSampledImageViewAspectMask(VK_IMAGE_ASPECT_STENCIL_BIT, GL_DEPTH_COMPONENT),
+              static_cast<VkImageAspectFlags>(VK_IMAGE_ASPECT_STENCIL_BIT));
+    EXPECT_EQ(VkTextureManager::ResolveSampledImageViewAspectMask(VK_IMAGE_ASPECT_COLOR_BIT, GL_STENCIL_INDEX),
+              static_cast<VkImageAspectFlags>(VK_IMAGE_ASPECT_COLOR_BIT));
+}
+
 TEST(DirectVulkanSanity, RenderPassExtentUsesSwapchainSizeOnlyForDefaultFramebuffer) {
     using MobileGL::MG_Backend::DirectVulkan::ResolveRenderPassFramebufferExtent;
 

MobileGL/MG_Test/State/NegativeApiErrorsTest.cpp

@@ -323,6 +323,47 @@ namespace {
         static_cast<void>(parameterBuffer);
     }
 
+    // A transform feedback name has two different truths and glDrawTransformFeedback used to ask
+    // for the wrong one. glGenTransformFeedbacks only RESERVES a name; the first
+    // glBindTransformFeedback is what creates the object (GL 4.6 core 13.2.1), and
+    // glIsTransformFeedback reports exactly that distinction. glDrawTransformFeedback's
+    // "id is not the name of a transform feedback object" INVALID_VALUE has to agree with
+    // glIsTransformFeedback, or a caller that picks an unused name the way
+    // KHR-GL4x.transform_feedback.api_errors_test does - increment until glIsTransformFeedback
+    // says false - gets a name the draw then accepts, and the draw falls through to a different
+    // error entirely (INVALID_OPERATION, "glEndTransformFeedback has never been called").
+    //
+    // The draw path itself needs a backend and a linked program before it reaches the name, which
+    // this GPU-free suite has neither of, so what is pinned here is the predicate pair the fix
+    // turns on: the two must not collapse back into one.
+    TEST_F(NegativeApiErrorsTest, ReservedTransformFeedbackNameIsNotYetAnObject) {
+        GLuint name = 0;
+        GenTransformFeedbacks(1, &name);
+        ASSERT_NE(name, 0u);
+        DrainErrors();
+
+        // Reserved, so it is a legal argument to glBindTransformFeedback...
+        EXPECT_TRUE(MG_State::pGLContext->ValidateTransformFeedbackName(name));
+        // ...but not an object yet, which is what a draw must key off.
+        EXPECT_FALSE(MG_State::pGLContext->IsTransformFeedbackObject(name));
+        EXPECT_EQ(IsTransformFeedback(name), GL_FALSE);
+
+        BindTransformFeedback(GL_TRANSFORM_FEEDBACK, name);
+        EXPECT_EQ(GetError(), GL_NO_ERROR);
+
+        EXPECT_TRUE(MG_State::pGLContext->ValidateTransformFeedbackName(name));
+        EXPECT_TRUE(MG_State::pGLContext->IsTransformFeedbackObject(name));
+        EXPECT_EQ(IsTransformFeedback(name), GL_TRUE);
+
+        // The default object is never "an object" by this predicate and is always drawable, so
+        // the draw path has to special-case it rather than reuse the answer directly.
+        EXPECT_FALSE(MG_State::pGLContext->IsTransformFeedbackObject(0));
+        EXPECT_TRUE(MG_State::pGLContext->ValidateTransformFeedbackName(0));
+
+        BindTransformFeedback(GL_TRANSFORM_FEEDBACK, 0);
+        DrainErrors();
+    }
+
     TEST_F(NegativeApiErrorsTest, TexStorage3DRejectsCompressedFormatsOnTexture3D) {
         GLuint texture = 0;
         GenTextures(1, &texture);

MobileGL/MG_Test/Texture/TextureTest.cpp

@@ -1651,6 +1651,43 @@ TEST_F(TextureTest, AnUncompressedRespecificationClearsTheCompressedTag) {
     EXPECT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
 }
 
+// GL_DEPTH_STENCIL_TEXTURE_MODE used to be a pure frontend shadow: stored, answered by
+// glGetTexParameter, and never shown to a backend. Sampling therefore always read the depth
+// aspect however the mode was set, which is the whole of
+// KHR-GL3x.packed_depth_stencil.stencil_texturing. Both backends pick the aspect up through the
+// texture-params version - DirectGLES re-emits glTexParameteri when it moves, DirectVulkan
+// rebuilds the sampled image view - so the version bump is the load-bearing part, and a
+// no-op write must not spend one (every bump costs DirectVulkan a view recreation).
+TEST_F(TextureTest, DepthStencilTextureModeIsBackendVisibleThroughTheParamsVersion) {
+    GLuint texture = 0;
+    MG_Impl::GLImpl::GenTextures(1, &texture);
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, texture);
+    MG_Impl::GLImpl::TexStorage2D(GL_TEXTURE_2D, 1, GL_DEPTH24_STENCIL8, 8, 8);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    const auto textureObject = MG_State::pGLContext->GetTextureObject(texture);
+    ASSERT_NE(textureObject, nullptr);
+    EXPECT_EQ(textureObject->GetDepthStencilTextureMode(), static_cast<GLenum>(GL_DEPTH_COMPONENT));
+
+    const Uint16 initialVersion = textureObject->GetTextureParamsVersion();
+    MG_Impl::GLImpl::TexParameteri(GL_TEXTURE_2D, GL_DEPTH_STENCIL_TEXTURE_MODE, GL_STENCIL_INDEX);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+    EXPECT_EQ(textureObject->GetDepthStencilTextureMode(), static_cast<GLenum>(GL_STENCIL_INDEX));
+    EXPECT_NE(textureObject->GetTextureParamsVersion(), initialVersion);
+
+    // Re-writing the value already in force is not a change and must not invalidate anything.
+    const Uint16 settledVersion = textureObject->GetTextureParamsVersion();
+    MG_Impl::GLImpl::TexParameteri(GL_TEXTURE_2D, GL_DEPTH_STENCIL_TEXTURE_MODE, GL_STENCIL_INDEX);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+    EXPECT_EQ(textureObject->GetTextureParamsVersion(), settledVersion);
+
+    // ...and going back to the depth aspect is a change again.
+    MG_Impl::GLImpl::TexParameteri(GL_TEXTURE_2D, GL_DEPTH_STENCIL_TEXTURE_MODE, GL_DEPTH_COMPONENT);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+    EXPECT_EQ(textureObject->GetDepthStencilTextureMode(), static_cast<GLenum>(GL_DEPTH_COMPONENT));
+    EXPECT_NE(textureObject->GetTextureParamsVersion(), settledVersion);
+}
+
 namespace {
     // 8x8 RGTC1: 2x2 blocks of 8 bytes, so the stored image is 32 bytes and one block row is 16.
     constexpr GLsizei kRgtc1Size8x8 = 32;