Don't write alpha when blitting to a framebuffer with emulated alpha.

src/libANGLE/renderer/gl/BlitGL.cpp

@@ -398,7 +398,8 @@ angle::Result BlitGL::blitColorBufferWithShader(const gl::Context *context,
                                                 const gl::Framebuffer *dest,
                                                 const gl::Rectangle &sourceAreaIn,
                                                 const gl::Rectangle &destAreaIn,
-                                                GLenum filter)
+                                                GLenum filter,
+                                                bool writeAlpha)
 {
     ANGLE_TRY(initializeResources(context));
 
@@ -488,6 +489,9 @@ angle::Result BlitGL::blitColorBufferWithShader(const gl::Context *context,
     ANGLE_TRY(scopedState.enter(context, destArea, ScopedGLState::KEEP_SCISSOR));
     scopedState.willUseTextureUnit(context, 0);
 
+    // Set the write color mask to potentially not write alpha
+    mStateManager->setColorMask(true, true, true, writeAlpha);
+
     // Set uniforms
     mStateManager->activeTexture(0);
     mStateManager->bindTexture(gl::TextureType::_2D, textureId);

src/libANGLE/renderer/gl/BlitGL.h

@@ -69,7 +69,8 @@ class BlitGL : angle::NonCopyable
                                             const gl::Framebuffer *dest,
                                             const gl::Rectangle &sourceArea,
                                             const gl::Rectangle &destArea,
-                                            GLenum filter);
+                                            GLenum filter,
+                                            bool writeAlpha);
 
     angle::Result copySubTexture(const gl::Context *context,
                                  TextureGL *source,

src/libANGLE/renderer/gl/FramebufferGL.cpp

@@ -767,6 +767,13 @@ angle::Result FramebufferGL::blit(const gl::Context *context,
             needManualColorBlit || (destSRGB && functions->isAtMostGL(gl::Version(4, 1)));
     }
 
+    // If the destination has an emulated alpha channel, we need to blit with a shader with alpha
+    // writes disabled.
+    if (mHasEmulatedAlphaAttachment)
+    {
+        needManualColorBlit = true;
+    }
+
     // Enable FRAMEBUFFER_SRGB if needed
     stateManager->setFramebufferSRGBEnabledForFramebuffer(context, true, this);
 
@@ -775,7 +782,8 @@ angle::Result FramebufferGL::blit(const gl::Context *context,
     {
         BlitGL *blitter = GetBlitGL(context);
         ANGLE_TRY(blitter->blitColorBufferWithShader(context, sourceFramebuffer, destFramebuffer,
-                                                     sourceArea, destArea, filter));
+                                                     sourceArea, destArea, filter,
+                                                     !mHasEmulatedAlphaAttachment));
         blitMask &= ~GL_COLOR_BUFFER_BIT;
     }