add encodeData() to SkImageEncoder, and add encoding to SkImage

src/image/SkImage_Gpu.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkImage_Base.h"
 #include "SkImagePriv.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
 #include "GrContext.h"
 #include "GrTexture.h"
 #include "SkGrPixelRef.h"
 
 class SkImage_Gpu : public SkImage_Base {
 public:
     SK_DECLARE_INST_COUNT(SkImage_Gpu)
 
     SkImage_Gpu(GrTexture*);
     virtual ~SkImage_Gpu();
 
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) SK_OVERRIDE;
     virtual GrTexture* onGetTexture() SK_OVERRIDE;
+    virtual bool getROPixels(SkBitmap*) const SK_OVERRIDE {
+        // TODO
+        return false;
+    }
 
     GrTexture* getTexture() { return fTexture; }
 
     void setTexture(GrTexture* texture);
 
 private:
     GrTexture*  fTexture;
     SkBitmap    fBitmap;
 
     typedef SkImage_Base INHERITED;
@@ skipping 46 matching lines @@
     return SkNEW_ARGS(SkImage_Gpu, (texture));
 }
 
 GrTexture* SkTextureImageGetTexture(SkImage* image) {
     return ((SkImage_Gpu*)image)->getTexture();
 }
 
 void SkTextureImageSetTexture(SkImage* image, GrTexture* texture) {
     ((SkImage_Gpu*)image)->setTexture(texture);
 }