DirectGLES: report the alpha added by the multisample widening as ONE

A three-channel format widened to four for a multisample target gains an
alpha channel the application never asked for, and it holds whatever the
draw that filled the texture happened to write there. GL says a format
without alpha reads back as 1.0, so KHR-GL33.texture_swizzle - which
fills such a texture by rendering vec4(r, g, b, 0.0) and then swizzles
red from alpha - read 0 where it expected the maximum.

Fold ONE into the texture's swizzle for exactly those textures, composed
with the swizzle the application set, so the promotion stays invisible.

MobileGL/MG_Backend/DirectGLES/Managers.cpp

@@ -2405,7 +2405,19 @@ namespace MobileGL::MG_Backend::DirectGLES {
                 MGLOG_D("%s(%s:%d) ES error %s", func, file, line, MG_Util::ConvertGLEnumToString(err).c_str());
             });
 
-            const auto& swizzleParams = stateTextureObject->GetAllSwizzleParams();
+            // A three-channel format widened to four for a multisample target (see
+            // NormalizePixelFormat) gains an alpha channel the frontend format does not have, and
+            // whatever the draw that filled it wrote there is not what GL would report: a format
+            // without alpha reads back as 1.0. Answer the ALPHA swizzle source with ONE so the
+            // promotion stays invisible, composed with the swizzle the application asked for.
+            Vec4<TextureSwizzleParam> swizzleParams = stateTextureObject->GetAllSwizzleParams();
+            if (TextureImpl::BackendTextureFormatAddsAlpha(stateTextureObject->GetFormat(), targetInternal)) {
+                for (SizeT channel = 0; channel < 4; ++channel) {
+                    if (swizzleParams[channel] == TextureSwizzleParam::Alpha) {
+                        swizzleParams[channel] = TextureSwizzleParam::One;
+                    }
+                }
+            }
             if (swizzleParams != m_cacheSwizzleParams) {
 #define SYNC_TEX_SWIZZLE_PARAM_IF_CHANGED(func, glEnum)                                                                \
     if (m_cacheSwizzleParams.func != swizzleParams.func) {                                                             \

MobileGL/MG_Backend/DirectGLES/Utils.cpp

@@ -177,6 +177,22 @@ namespace MobileGL::MG_Backend::DirectGLES {
         Bool ShouldUseCaveatRenderbufferFormat(TextureInternalFormat internalFormat) {
             return ShouldUseCaveatFormat(internalFormat, GetRenderbufferFormatCapabilityTargetIndex());
         }
+
+        Bool BackendTextureFormatAddsAlpha(TextureInternalFormat internalFormat, TextureTarget target) {
+            const SizeT targetIndex =
+                target == TextureTarget::Unknown ? kFormatCapabilityTargetCount : GetFormatCapabilityTargetIndex(target);
+            if (!TargetRequiresRenderableFormat(targetIndex)) {
+                return false;
+            }
+            if (pActiveBackendObject && !ShouldUseCaveatFormat(internalFormat, targetIndex)) {
+                return false;
+            }
+            const GLenum requestedInternalFormat = MG_Util::ConvertTextureInternalFormatToGLEnum(internalFormat);
+            const Flags<PixelFormatNormalizeOptionBit> options =
+                GetRuntimeFallbackNormalizeOptions(requestedInternalFormat,
+                                                   GetRenderTargetNormalizeOptions(targetIndex));
+            return static_cast<Bool>(options & PixelFormatNormalizeOptionBit::NoThreeChannelRenderTarget);
+        }
     } // namespace TextureImpl
     namespace PrgramImpl {
         String ProcessOutColorLocations(const String& glslCode) {

MobileGL/MG_Backend/DirectGLES/Utils.h

@@ -40,6 +40,11 @@ namespace MobileGL::MG_Backend::DirectGLES {
         void GenerateRenderbufferFormatInfo(TextureInternalFormat internalFormat, GLenum* outInternalFormat,
                                             GLenum* outFormat, GLenum* outType);
         Bool ShouldUseCaveatTextureFormat(TextureInternalFormat internalFormat, TextureTarget target);
+
+        // True when the format the texture is actually created with has an alpha channel the
+        // frontend format does not (the three-channel multisample widening). GL reads such a
+        // channel back as 1.0, so any swizzle source of ALPHA has to be answered with ONE.
+        Bool BackendTextureFormatAddsAlpha(TextureInternalFormat internalFormat, TextureTarget target);
         Bool ShouldUseCaveatRenderbufferFormat(TextureInternalFormat internalFormat);
     } // namespace TextureImpl