Bug 987010 - Fix the way that the presence of an alpha channel on a canvas drawing buffer is propagated - r=jrmuizel

content/canvas/src/CanvasRenderingContext2D.cpp

@@ -4255,6 +4255,7 @@ CanvasRenderingContext2D::GetCanvasLayer(nsDisplayListBuilder* aBuilder,
   }
 
   data.mSize = nsIntSize(mWidth, mHeight);
+  data.mHasAlpha = !mOpaque;
 
   canvasLayer->Initialize(data);
   uint32_t flags = mOpaque ? Layer::CONTENT_OPAQUE : 0;

content/canvas/src/WebGLContext.cpp

@@ -893,6 +893,7 @@ WebGLContext::GetCanvasLayer(nsDisplayListBuilder* aBuilder,
     CanvasLayer::Data data;
     data.mGLContext = gl;
     data.mSize = nsIntSize(mWidth, mHeight);
+    data.mHasAlpha = gl->Caps().alpha;
     data.mIsGLAlphaPremult = IsPremultAlpha();
 
     canvasLayer->Initialize(data);

gfx/layers/Layers.h

@@ -1869,6 +1869,7 @@ public:
       , mStream(nullptr)
       , mTexID(0)
       , mSize(0,0)
+      , mHasAlpha(false)
       , mIsGLAlphaPremult(false)
     { }
 
@@ -1885,6 +1886,9 @@ public:
     // The size of the canvas content
     nsIntSize mSize;
 
+    // Whether the canvas drawingbuffer has an alpha channel.
+    bool mHasAlpha;
+
     // Whether mGLContext contains data that is alpha-premultiplied.
     bool mIsGLAlphaPremult;
   };

gfx/layers/client/ClientCanvasLayer.cpp

@@ -55,11 +55,14 @@ ClientCanvasLayer::Initialize(const Data& aData)
   if (mGLContext) {
     GLScreenBuffer* screen = mGLContext->Screen();
 
-    SurfaceCaps caps = screen->Caps();
+    SurfaceCaps caps;
     if (mStream) {
       // The screen caps are irrelevant if we're using a separate stream
-      caps = GetContentFlags() & CONTENT_OPAQUE ? SurfaceCaps::ForRGB() : SurfaceCaps::ForRGBA();
+      caps = aData.mHasAlpha ? SurfaceCaps::ForRGBA() : SurfaceCaps::ForRGB();
+    } else {
+      caps = screen->Caps();
     }
+    MOZ_ASSERT(caps.alpha == aData.mHasAlpha);
 
     SurfaceStreamType streamType =
         SurfaceStream::ChooseGLStreamType(SurfaceStream::OffMainThread,