Adding public API method on SkImage for extracting the GPU texture handle.

src/image/SkImage_Base.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_Base_DEFINED
 #define SkImage_Base_DEFINED
 
 #include "SkImage.h"
 
 class SkImage_Base : public SkImage {
 public:
     SkImage_Base(int width, int height) : INHERITED(width, height) {}
 
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) = 0;
+    virtual GrTexture* onGetTexture() { return NULL; }
 
 private:
     typedef SkImage INHERITED;
 };
 
 #endif