Bug 757380 - Check clip before creating group target. r=Bas

gfx/layers/basic/BasicLayers.cpp

@@ -1976,7 +1976,8 @@ BasicLayerManager::PaintLayer(gfxContext* aTarget,
 
   nsRefPtr<gfxContext> groupTarget;
   nsRefPtr<gfxASurface> untransformedSurface;
-  if (!is2D) {
+  bool clipIsEmpty = aTarget->GetClipExtents().IsEmpty();
+  if (!is2D && !clipIsEmpty) {
     untransformedSurface = 
       gfxPlatform::GetPlatform()->CreateOffscreenSurface(gfxIntSize(bounds.width, bounds.height), 
                                                          gfxASurface::CONTENT_COLOR_ALPHA);
@@ -1994,7 +1995,7 @@ BasicLayerManager::PaintLayer(gfxContext* aTarget,
     }
     untransformedSurface->SetDeviceOffset(gfxPoint(-bounds.x, -bounds.y));
     groupTarget = new gfxContext(untransformedSurface);
-  } else if (needsGroup) {
+  } else if (needsGroup && !clipIsEmpty) {
     groupTarget = PushGroupForLayer(aTarget, aLayer, aLayer->GetEffectiveVisibleRegion(),
                                     &needsClipToVisibleRegion);
   } else {
@@ -2051,9 +2052,7 @@ BasicLayerManager::PaintLayer(gfxContext* aTarget,
 
       // Temporary fast fix for bug 725886
       // Revert these changes when 725886 is ready
-      gfxRect clipExtents;
-      clipExtents = aTarget->GetClipExtents();
-      if (!clipExtents.IsEmpty()) {
+      if (!clipIsEmpty) {
         gfxPoint offset;
         bool dontBlit = needsClipToVisibleRegion || mTransactionIncomplete ||
                           aLayer->GetEffectiveOpacity() != 1.0f;