Add support for multiple frames in SkCodec

src/codec/SkSwizzler.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.
  */
 
 #ifndef SkSwizzler_DEFINED
 #define SkSwizzler_DEFINED
 
@@ skipping 60 matching lines @@
      *  this allows us to apply a transparency mask to pixels after swizzling.
      */
     int sampleX() const { return fSampleX; }
 
     /**
      *  Returns the actual number of pixels written to destination memory, taking
      *  scaling, subsetting, and partial frames into account.
      */
     int swizzleWidth() const { return fSwizzleWidth; }
 
+    /**
+     *  Returns the byte offset at which we write to destination memory, taking
+     *  scaling, subsetting, and partial frames into account.
+     */
+    size_t swizzleOffsetBytes() const { return fDstOffsetBytes; }
+
 private:
 
     /**
      *  Method for converting raw data to Skia pixels.
      *  @param dstRow Row in which to write the resulting pixels.
      *  @param src Row of src data, in format specified by SrcConfig
      *  @param dstWidth Width in pixels of the destination
      *  @param bpp if bitsPerPixel % 8 == 0, deltaSrc is bytesPerPixel
      *             else, deltaSrc is bitsPerPixel
      *  @param deltaSrc bpp * sampleX
@@ skipping 110 matching lines @@
                                           //     fBPP is bitsPerPixel
     const int           fDstBPP;          // Bytes per pixel for the destination color type
 
     SkSwizzler(RowProc fastProc, RowProc proc, const SkPMColor* ctable, int srcOffset,
             int srcWidth, int dstOffset, int dstWidth, int srcBPP, int dstBPP);
 
     int onSetSampleX(int) override;
 
 };
 #endif // SkSwizzler_DEFINED