Bug 1007897 - Release mSurface reference earlier to reduce copies and peak memory. r=mattwoodrow

--HG--
extra : rebase_source : 2f2a46c66e41010a5e8afb4d9d6e95d0f27bd763

gfx/layers/CopyableCanvasLayer.cpp

@@ -90,6 +90,7 @@ CopyableCanvasLayer::UpdateTarget(DrawTarget* aDestTarget)
       aDestTarget->CopySurface(mSurface,
                                IntRect(0, 0, mBounds.width, mBounds.height),
                                IntPoint(0, 0));
+      mSurface = nullptr;
     }
     return;
   }