[Feat] (MG_Impl, MG_State, MG_Util): attach a buffer texture to a range of its buffer

glTexBufferRange, glTextureBuffer and glTextureBufferRange were all stubs, so a
buffer texture could only ever be attached through glTexBuffer -- by binding, and
always to the whole buffer.

Give the buffer texture the window it is supposed to address. The non-range forms
record it as offset 0 with a whole-buffer sentinel rather than the size the buffer
happens to have, so a later respecify keeps being followed instead of freezing the
texture at yesterday's size. All four entry points now share one attach path,
differing only in how they name the texture: by binding for the target forms, by
name for the DSA ones.

Both backends honour the window: DirectVulkan offsets and clamps the buffer view,
DirectGLES uses glTexBufferRange when the texture names a sub-range and keeps
plain glTexBuffer for the whole-buffer case, which also works on a driver without
the range entry point.

GL_TEXTURE_BUFFER_OFFSET_ALIGNMENT reported 0 with a comment explaining that the
range entry points were stubbed. It now reports what the device actually requires
-- minTexelBufferOffsetAlignment on Vulkan, the driver's own value on GLES -- and
the range entry points enforce it. Zero was never a legal answer; the minimum is
1, and an application that trusted it would have built unaligned offsets.

MobileGL/MG_Util/BackendLoaders/OpenGL/Loader.cpp

@@ -1056,6 +1056,10 @@ namespace MobileGL::MG_Util::BackendLoader {
         caps.MaxComputeWorkGroupInvocations = maxComputeWorkGroupInvocations;
         caps.MaxShaderStorageBufferBindings = maxShaderStorageBufferBindings;
         caps.MaxTextureBufferSize = maxTextureBufferSize;
+        GLint textureBufferOffsetAlignment = 1;
+        glGetIntegerv(GL_TEXTURE_BUFFER_OFFSET_ALIGNMENT, &textureBufferOffsetAlignment);
+        while (glGetError() != GL_NO_ERROR) {}
+        caps.TextureBufferOffsetAlignment = std::max(1, textureBufferOffsetAlignment);
         caps.MaxUniformBufferBindings = maxUniformBufferBindings;
         caps.MaxUniformBlockSize = maxUniformBlockSize;
         caps.MaxImageUnits = maxImageUnits;

MobileGL/MG_Util/BackendLoaders/OpenGL/Loader.h

@@ -1117,6 +1117,8 @@ namespace MobileGL {
             Int MaxComputeWorkGroupInvocations = 128;
             Int MaxShaderStorageBufferBindings = 8;
             Int MaxTextureBufferSize = 65536;
+            // GL_TEXTURE_BUFFER_OFFSET_ALIGNMENT; 1 means the offset is unconstrained.
+            Int TextureBufferOffsetAlignment = 1;
             Int MaxUniformBufferBindings = 24;
             Int MaxUniformBlockSize = 16384;
             Int MaxImageUnits = 8;

MobileGL/MG_Util/BackendLoaders/Vulkan/Loader.cpp

@@ -177,6 +177,8 @@ namespace MobileGL::MG_Util::BackendLoader {
         caps.MaxComputeWorkGroupInvocations = static_cast<Int>(p.limits.maxComputeWorkGroupInvocations);
         caps.MaxShaderStorageBufferBindings = static_cast<Int>(p.limits.maxDescriptorSetStorageBuffers);
         caps.MaxTextureBufferSize = static_cast<Int>(p.limits.maxTexelBufferElements);
+        caps.TextureBufferOffsetAlignment =
+            static_cast<Int>(std::max<VkDeviceSize>(1, p.limits.minTexelBufferOffsetAlignment));
         caps.MaxUniformBufferBindings = static_cast<Int>(p.limits.maxDescriptorSetUniformBuffers);
         caps.MaxUniformBlockSize = static_cast<Int>(p.limits.maxUniformBufferRange);
         caps.MaxImageUnits = static_cast<Int>(p.limits.maxPerStageDescriptorStorageImages);
@@ -268,6 +270,8 @@ namespace MobileGL::MG_Util::BackendLoader {
         caps.MaxComputeWorkGroupInvocations = static_cast<Int>(properties.limits.maxComputeWorkGroupInvocations);
         caps.MaxShaderStorageBufferBindings = static_cast<Int>(properties.limits.maxDescriptorSetStorageBuffers);
         caps.MaxTextureBufferSize = static_cast<Int>(properties.limits.maxTexelBufferElements);
+        caps.TextureBufferOffsetAlignment =
+            static_cast<Int>(std::max<VkDeviceSize>(1, properties.limits.minTexelBufferOffsetAlignment));
         caps.MaxUniformBufferBindings = static_cast<Int>(properties.limits.maxDescriptorSetUniformBuffers);
         caps.MaxUniformBlockSize = static_cast<Int>(properties.limits.maxUniformBufferRange);
         caps.MaxImageUnits = static_cast<Int>(properties.limits.maxPerStageDescriptorStorageImages);

MobileGL/MG_Util/BackendLoaders/Vulkan/Loader.h

@@ -54,6 +54,8 @@ namespace MobileGL {
             Int MaxComputeWorkGroupInvocations = 128;
             Int MaxShaderStorageBufferBindings = 8;
             Int MaxTextureBufferSize = 65536;
+            // GL_TEXTURE_BUFFER_OFFSET_ALIGNMENT; 1 means the offset is unconstrained.
+            Int TextureBufferOffsetAlignment = 1;
             Int MaxUniformBufferBindings = 24;
             Int MaxUniformBlockSize = 16384;
             Int MaxImageUnits = 8;