Allow creating multiple scanline decoders.

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 SkCodec* NewFromStream(SkStream*);
     static bool IsPng(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;
     int                         fNumberPasses;
     bool                        fReallyHasAlpha;
     int                         fBitDepth;
 
     SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop, int);
-    ~SkPngCodec();
 
 
     // Helper to set up swizzler and color table. Also calls png_read_update_info.
     Result initializeSwizzler(const SkImageInfo& requestedInfo, void* dst,
                               size_t rowBytes, const Options&, SkPMColor*, int* ctableCount);
 
     // Calls rewindIfNeeded and returns true if the decoder can continue.
     bool handleRewind();
-    bool decodePalette(bool premultiply, int bitDepth, int* ctableCount);
+    bool decodePalette(bool premultiply, int* ctableCount);
     void finish();
     void destroyReadStruct();
 
     friend class SkPngScanlineDecoder;
     friend class SkPngInterlacedScanlineDecoder;
 
     typedef SkCodec INHERITED;
 };