Move all knowledge of X sampling into SkScaledCodec

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 "SkColorTable.h"
 #include "SkEncodedFormat.h"
@@ skipping 23 matching lines @@
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
     bool onRewind() override;
     bool onReallyHasAlpha() const final;
 
     // Helper to set up swizzler and color table. Also calls png_read_update_info.
     Result initializeSwizzler(const SkImageInfo& requestedInfo, const Options&,
                               SkPMColor*, int* ctableCount);
+    SkSampler* getSampler() override { return fSwizzler; }
 
     SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop, int, int);
 
     png_structp png_ptr() { return fPng_ptr; }
     SkSwizzler* swizzler() { return fSwizzler; }
     SkSwizzler::SrcConfig srcConfig() const { return fSrcConfig; }
     int numberPasses() const { return fNumberPasses; }
 
     enum AlphaState {
         // This class has done no decoding, or threw away its knowledge (in
@@ skipping 20 matching lines @@
     const int                   fNumberPasses;
     int                         fBitDepth;
 
     AlphaState                  fAlphaState;
 
     bool decodePalette(bool premultiply, int* ctableCount);
     void destroyReadStruct();
 
     typedef SkCodec INHERITED;
 };