Bug 771219 - Restore display items when drawing a thebes layer. r=roc

layout/base/FrameLayerBuilder.cpp

@@ -2582,6 +2582,12 @@ FrameLayerBuilder::DrawThebesLayer(ThebesLayer* aLayer,
       break;
   }
 
+  {
+    ThebesLayerItemsEntry* entry =
+      builder->LayerBuilder()->mThebesLayerItems.GetEntry(aLayer);
+    items.SwapElements(entry->mItems);
+  }
+
   if (setClipRect) {
     aContext->Restore();
   }

layout/base/FrameLayerBuilder.h

@@ -218,6 +218,8 @@ public:
   /**
    * This callback must be provided to EndTransaction. The callback data
    * must be the nsDisplayListBuilder containing this FrameLayerBuilder.
+   * This function can be called multiple times in a row to draw
+   * different regions.
    */
   static void DrawThebesLayer(ThebesLayer* aLayer,
                               gfxContext* aContext,