Discard atlas after every MultiPictureDraw::draw

src/core/SkMultiPictureDraw.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #if SK_SUPPORT_GPU
 #include "GrLayerHoister.h"
 #include "GrRecordReplaceDraw.h"
@@ skipping 164 matching lines @@
         } else
 #endif
         {
             canvas->drawPicture(picture, &data.fMatrix, data.fPaint);
         }
     }
 
 #ifndef SK_IGNORE_GPU_LAYER_HOISTING
     GrLayerHoister::UnlockLayers(context, atlasedNeedRendering);
     GrLayerHoister::UnlockLayers(context, atlasedRecycled);
+#if !GR_CACHE_HOISTED_LAYERS
+    GrLayerHoister::PurgeCache(context);
+#endif
 #endif
 }