Create a scanline decoder without creating a codec

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:
-    // Assumes IsPng was called and returned true.
+    static bool IsPng(SkStream*);
+
+    // Assume IsPng was called and returned true.
     static SkCodec* NewFromStream(SkStream*);
-    static bool IsPng(SkStream*);
+    static SkScanlineDecoder* NewSDFromStream(SkStream*);
 
     virtual ~SkPngCodec();
 
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
-    SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo, const Options& options,
-                                            SkPMColor ctable[], int* ctableCount) override;
     bool onReallyHasAlpha() const override { return fReallyHasAlpha; }
 private:
     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;
@@ skipping 11 matching lines @@
     // Calls rewindIfNeeded and returns true if the decoder can continue.
     bool handleRewind();
     bool decodePalette(bool premultiply, int* ctableCount);
     void destroyReadStruct();
 
     friend class SkPngScanlineDecoder;
     friend class SkPngInterlacedScanlineDecoder;
 
     typedef SkCodec INHERITED;
 };