Clip buffer damage to viewport bounds

bug:27287946

Change-Id: Ief3ae9c2dd92196b7d09f1b9fadf009eb228d80a
diff --git a/libs/hwui/renderstate/OffscreenBufferPool.h b/libs/hwui/renderstate/OffscreenBufferPool.h
index 94155ef..089f131 100644
--- a/libs/hwui/renderstate/OffscreenBufferPool.h
+++ b/libs/hwui/renderstate/OffscreenBufferPool.h
@@ -48,6 +48,8 @@
 
     Rect getTextureCoordinates();
 
+    void dirty(Rect dirtyArea);
+
     // must be called prior to rendering, to construct/update vertex buffer
     void updateMeshFromRegion();