Adding optimization to avoid image copy in SkSurface copy on write when content is discardable

include/core/SkSurface.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSurface_DEFINED
 #define SkSurface_DEFINED
 
@@ skipping 61 matching lines @@
      *  surface. Each time the content is changed changed, either by drawing
      *  into this surface, or explicitly calling notifyContentChanged()) this
      *  method will return a new value.
      *
      *  If this surface is empty (i.e. has a zero-dimention), this will return
      *  0.
      */
     uint32_t generationID();
 
     /**
-     *  Call this if the contents have changed. This will (lazily) force a new
+     *  Modes that can be passed to notifyContentWillChange
+     */
+    enum ContentChangeMode {
+        /** 
+         *  Use this mode if it is known that the upcoming content changes will
+         *  clear or overwrite prior contents, thus making them discardable.
+         */
+        kDiscard_ContentChangeMode,
+        /** 
+         *  Use this mode if prior surface contents need to be preserved or
+         *  if in doubt.
+         */
+        kRetain_ContentChangeMode,
+    };
+
+    /**
+     *  Call this if the contents are about to change. This will (lazily) force a new
      *  value to be returned from generationID() when it is called next.
      */
-    void notifyContentChanged();
+    void notifyContentWillChange(ContentChangeMode mode);
 
     /**
      *  Return a canvas that will draw into this surface. This will always
      *  return the same canvas for a given surface, and is manged/owned by the
      *  surface. It should not be used when its parent surface has gone out of
      *  scope.
      */
     SkCanvas* getCanvas();
 
     /**
@@ skipping 37 matching lines @@
 
 private:
     const int   fWidth;
     const int   fHeight;
     uint32_t    fGenerationID;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif