Bug 770144 - Restrict ThebesLayer invalidations to the visible region. r=roc

layout/base/FrameLayerBuilder.cpp

@@ -690,6 +690,7 @@ InvalidatePostTransformRegion(ThebesLayer* aLayer, const nsIntRegion& aRegion,
   // to the ThebesLayer's own coordinates
   nsIntRegion rgn = aRegion;
   rgn.MoveBy(-aTranslation);
+  rgn.And(rgn, aLayer->GetVisibleRegion());
   aLayer->InvalidateRegion(rgn);
 #ifdef DEBUG_INVALIDATIONS
   nsCAutoString str;