Purge cached resources on SkImage destruction.

include/core/SkImage.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 SkImage_DEFINED
 #define SkImage_DEFINED
 
@@ skipping 310 matching lines @@
     bool asLegacyBitmap(SkBitmap*, LegacyBitmapMode) const;
     
     /**
      *  Returns true if the image is backed by an image-generator or other src that creates
      *  (and caches) its pixels / texture on-demand.
      */
     bool isLazyGenerated() const;
 
 protected:
     SkImage(int width, int height, uint32_t uniqueID);
+    virtual ~SkImage();
 
 private:
     const int       fWidth;
     const int       fHeight;
     const uint32_t  fUniqueID;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif