Move Layer creation/deletion into the RenderPipeline.

Test: refactoring. existing tests still pass.
Change-Id: I032c33896a0cb74c91e2a913a584373518466b88
diff --git a/libs/hwui/RenderNode.h b/libs/hwui/RenderNode.h
index a0679b1..da93c13 100644
--- a/libs/hwui/RenderNode.h
+++ b/libs/hwui/RenderNode.h
@@ -199,6 +199,7 @@
     }
     OffscreenBuffer* getLayer() const { return mLayer; }
     OffscreenBuffer** getLayerHandle() { return &mLayer; } // ugh...
+    void setLayer(OffscreenBuffer* layer) { mLayer = layer; }
 
     // Note: The position callbacks are relying on the listener using
     // the frameNumber to appropriately batch/synchronize these transactions.