Add scanline decoding to SkCodec.

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 "SkImageInfo.h"
+#include "SkRefCnt.h"
+#include "SkSwizzler.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 SkScanlineDecoder;
 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;
+    SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo,
+                                            const SkIRect& origSubset) SK_OVERRIDE;
+    bool onReallyHasAlpha() const SK_OVERRIDE { return fReallyHasAlpha; }
 private:
-    png_structp             fPng_ptr;
-    png_infop               fInfo_ptr;
+    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       fSc;
+    int                         fNumberPasses;
+    bool                        fReallyHasAlpha;
 
     SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop);
     ~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);
+    bool decodePalette(bool premultiply);
+    Result finish();
+
+    friend class SkPngScanlineDecoder;
+
     typedef SkCodec INHERITED;
 };