Bug 768079; mask layers force an intermediate surface for their containers. r=roc

gfx/layers/Layers.cpp

@@ -467,8 +467,10 @@ ContainerLayer::DefaultComputeEffectiveTransforms(const gfx3DMatrix& aTransformT
           /* We can't (easily) forward our transform to children with a non-empty clip
            * rect since it would need to be adjusted for the transform. See
            * the calculations performed by CalculateScissorRect above.
+           * Nor for a child with a mask layer.
            */
-          if (clipRect && !clipRect->IsEmpty() && !child->GetVisibleRegion().IsEmpty()) {
+          if ((clipRect && !clipRect->IsEmpty() && !child->GetVisibleRegion().IsEmpty()) ||
+              child->GetMaskLayer()) {
             useIntermediateSurface = true;
             break;
           }