Option for SkCodec to treat dst as all zeroes.

src/codec/SkCodec_libpng.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkImageInfo.h"
 
 extern "C" {
     // FIXME: I'd like to force all platforms to use the same decoder, but this
     // means an extra dependency on Mac/Win.
     #include "png.h"
 }
 
 class SkStream;
 
 class SkPngCodec : public SkCodec {
 public:
     // Assumes IsPng was called and returned true.
     static SkCodec* NewFromStream(SkStream*);
     static bool IsPng(SkStream*);
 protected:
-    Result onGetPixels(const SkImageInfo&, void*, size_t, SkPMColor*, int*) SK_OVERRIDE;
+    Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
+            SK_OVERRIDE;
 private:
     png_structp             fPng_ptr;
     png_infop               fInfo_ptr;
 
     SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop);
     ~SkPngCodec();
 
     typedef SkCodec INHERITED;
 };