When mix-blending, only copy intersecting backdrop pixels. (bug 1235995 part 2, r=mattwoodrow)

gfx/layers/opengl/OGLShaderProgram.h

@@ -52,6 +52,7 @@ public:
     LayerTransform = 0,
     LayerTransformInverse,
     MaskTransform,
+    BackdropTransform,
     LayerRects,
     MatrixProj,
     TextureTransform,
@@ -352,6 +353,10 @@ public:
     SetMatrixUniform(KnownUniform::MaskTransform, aMatrix);
   }
 
+  void SetBackdropTransform(const gfx::Matrix4x4& aMatrix) {
+    SetMatrixUniform(KnownUniform::BackdropTransform, aMatrix);
+  }
+
   void SetDEAAEdges(const gfx::Point3D* aEdges) {
     SetArrayUniform(KnownUniform::SSEdges, 4, aEdges);
   }