Bug 940959 - Use alpha textures for YCbCr planes with D3D11. r=nrc

gfx/layers/d3d11/CompositorD3D11.fx

@@ -185,9 +185,9 @@ float4 CalculateYCbCrColor(const float2 aTexCoords)
   float4 yuv;
   float4 color;
 
-  yuv.r = tCr.Sample(sSampler, aTexCoords).r - 0.5;
-  yuv.g = tY.Sample(sSampler, aTexCoords).r - 0.0625;
-  yuv.b = tCb.Sample(sSampler, aTexCoords).r - 0.5;
+  yuv.r = tCr.Sample(sSampler, aTexCoords).a - 0.5;
+  yuv.g = tY.Sample(sSampler, aTexCoords).a - 0.0625;
+  yuv.b = tCb.Sample(sSampler, aTexCoords).a - 0.5;
 
   color.r = yuv.g * 1.164 + yuv.r * 1.596;
   color.g = yuv.g * 1.164 - 0.813 * yuv.r - 0.391 * yuv.b;

gfx/layers/d3d11/TextureD3D11.cpp

@@ -496,7 +496,7 @@ DeprecatedTextureHostYCbCrD3D11::UpdateImpl(const SurfaceDescriptor& aImage,
   initData.pSysMem = yuvDeserializer.GetYData();
   initData.SysMemPitch = yuvDeserializer.GetYStride();
 
-  CD3D11_TEXTURE2D_DESC desc(DXGI_FORMAT_R8_UNORM, size.width, size.height,
+  CD3D11_TEXTURE2D_DESC desc(DXGI_FORMAT_A8_UNORM, size.width, size.height,
                              1, 1, D3D11_BIND_SHADER_RESOURCE,
                              D3D11_USAGE_IMMUTABLE);