Bug 952051 - Change the GetOperator call in FlushGroup to take into account the mix-blend-mode operator for the group. r=roc

Create a new GetEffectiveOperator function that also takes into acount the mix-blend-mode operator for basic layers. Replace logic that chooses between the effective mix blend mode and the operator returned by GetOperator, with calls to this function.

gfx/layers/basic/BasicLayersImpl.cpp

@@ -135,6 +135,18 @@ ToData(Layer* aLayer)
   return static_cast<BasicImplData*>(aLayer->ImplData());
 }
 
+gfx::CompositionOp
+GetEffectiveOperator(Layer* aLayer)
+{
+  CompositionOp op = aLayer->GetEffectiveMixBlendMode();
+
+  if (op != CompositionOp::OP_OVER) {
+    return op;
+  }
+
+  return ToData(aLayer)->GetOperator();
+}
+
 ShadowableLayer*
 ToShadowable(Layer* aLayer)
 {