Add subsetting to SkScanlineDecoder

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"
 #include "SkImageInfo.h"
 #include "SkRefCnt.h"
 #include "SkSwizzler.h"
 
 #ifdef SKIA_PNG_PREFIXED
     // this must proceed png.h
     #include "pngprefix.h"
 #endif
 #include "png.h"
 
-class SkScanlineDecoder;
 class SkStream;
 
 class SkPngCodec : public SkCodec {
 public:
     static bool IsPng(SkStream*);
 
     // Assume IsPng was called and returned true.
     static SkCodec* NewFromStream(SkStream*);
-    static SkScanlineDecoder* NewSDFromStream(SkStream*);
 
     virtual ~SkPngCodec();
 
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
     bool onRewind() override;
     uint32_t onGetFillValue(SkColorType colorType, SkAlphaType alphaType) const override;
     bool onReallyHasAlpha() const final;
@@ skipping 30 matching lines @@
     png_structp                 fPng_ptr;
     png_infop                   fInfo_ptr;
 
     // These are stored here so they can be used both by normal decoding and scanline decoding.
     SkAutoTUnref<SkColorTable>  fColorTable;    // May be unpremul.
     SkAutoTDelete<SkSwizzler>   fSwizzler;
 
     SkSwizzler::SrcConfig       fSrcConfig;
     const int                   fNumberPasses;
     int                         fBitDepth;
-
     AlphaState                  fAlphaState;
 
     bool decodePalette(bool premultiply, int* ctableCount);
     void destroyReadStruct();
 
     typedef SkCodec INHERITED;
 };