Bug 858550 - Don't transform fixed layer descendents multiple times. r=nrc

Once the transform for async scrolling has been applied to a fixed layer, it
need not be applied to its descendents. Make sure to return after applying
the transform.

gfx/layers/ipc/CompositorParent.cpp

@@ -663,6 +663,10 @@ CompositorParent::TransformFixedLayers(Layer* aLayer,
       transformedClipRect.MoveBy(translation.x, translation.y);
       shadow->SetShadowClipRect(&transformedClipRect);
     }
+
+    // The transform has now been applied, so there's no need to iterate over
+    // child layers.
+    return;
   }
 
   for (Layer* child = aLayer->GetFirstChild();