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

sky/engine/core/painting/DrawLooper.idl

Index: sky/engine/core/painting/DrawLooper.idl
diff --git a/sky/engine/core/painting/Picture.idl b/sky/engine/core/painting/DrawLooper.idl
similarity index 87%
copy from sky/engine/core/painting/Picture.idl
copy to sky/engine/core/painting/DrawLooper.idl
index 0f618fdef72cafeff11960cb00f1fb24a959a6b0..8fbda9d1f82e171b595c0d03c1eb0dc652f24f23 100644
--- a/sky/engine/core/painting/Picture.idl
+++ b/sky/engine/core/painting/DrawLooper.idl
@@ -1,5 +1,6 @@
 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-interface Picture {
+
+interface DrawLooper {
 };