Bug 914457 - Part 1: Use an empty clip rect for layers with an empty visible region

- Layer::CalculateScissorRect no longer asserts when all children of a
  3d transformed layer have an empty visible region.

--HG--
extra : rebase_source : e05b1e12d88d36a7fedd20fc50c1f02a5370d7fc

gfx/layers/Layers.cpp

@@ -700,6 +700,13 @@ Layer::CalculateScissorRect(const RenderTargetIntRect& aCurrentScissorRect)
     return currentClip;
   }
 
+  if (GetVisibleRegion().IsEmpty()) {
+    // When our visible region is empty, our parent may not have created the
+    // intermediate surface that we would require for correct clipping; however,
+    // this does not matter since we are invisible.
+    return RenderTargetIntRect(currentClip.TopLeft(), RenderTargetIntSize(0, 0));
+  }
+
   const RenderTargetIntRect clipRect =
     ViewAs<RenderTargetPixel>(*GetEffectiveClipRect(),
                               PixelCastJustification::RenderTargetIsParentLayerForRoot);