Sky: Add a DrawLooper interface to the painting API to be used for shadows.

sky/engine/core/painting/DrawLooperAddLayerCallback.idl

Index: sky/engine/core/painting/DrawLooperAddLayerCallback.idl
diff --git a/sky/engine/core/layout/LayoutCallback.idl b/sky/engine/core/painting/DrawLooperAddLayerCallback.idl
similarity index 67%
copy from sky/engine/core/layout/LayoutCallback.idl
copy to sky/engine/core/painting/DrawLooperAddLayerCallback.idl
index b6618f90c1c954c9bd239c3d753469120955b0d1..6b62dc3f562521832c5f0b6652461b8d24d4595d 100644
--- a/sky/engine/core/layout/LayoutCallback.idl
+++ b/sky/engine/core/painting/DrawLooperAddLayerCallback.idl
@@ -2,6 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-callback interface LayoutCallback {
-    void handleEvent();
+callback interface DrawLooperAddLayerCallback {
+    void handleEvent(Paint paint);
 };