Bug 677095. Compute reasonable destination alpha values with component-alpha blending in D3D10. r=bas

--HG--
extra : rebase_source : b0264c52df9a37334d9524e1fdd1f15e4e4df866

gfx/layers/d3d10/LayerManagerD3D10.fx

@@ -48,8 +48,8 @@ BlendState ComponentAlphaBlend
   SrcBlend = One;
   DestBlend = Inv_Src1_Color;
   BlendOp = Add;
-  SrcBlendAlpha = Zero;
-  DestBlendAlpha = One;
+  SrcBlendAlpha = Src1_Alpha;
+  DestBlendAlpha = Inv_Src1_Alpha;
   BlendOpAlpha = Add;
   RenderTargetWriteMask[0] = 0x0F; // All
 };
@@ -172,6 +172,7 @@ PS_OUTPUT ComponentAlphaShader(const VS_OUTPUT aVertex) : SV_Target
 
   result.vSrc = tRGB.Sample(LayerTextureSamplerLinear, aVertex.vTexCoords);
   result.vAlpha = 1.0 - tRGBWhite.Sample(LayerTextureSamplerLinear, aVertex.vTexCoords) + result.vSrc;
+  result.vAlpha.a = result.vAlpha.g;
   result.vSrc *= fLayerOpacity;
   result.vAlpha *= fLayerOpacity;
   return result;