Add subsetting to SkScanlineDecoder

src/codec/SkMaskSwizzler.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 SkMaskSwizzler_DEFINED
 #define SkMaskSwizzler_DEFINED
 
 #include "SkMasks.h"
@@ skipping 10 matching lines @@
 class SkMaskSwizzler : public SkSampler {
 public:
 
     /*
      * Create a new swizzler
      * @param masks Unowned pointer to helper class
      */
     static SkMaskSwizzler* CreateMaskSwizzler(const SkImageInfo& dstInfo,
                                               const SkImageInfo& srcInfo,
                                               SkMasks* masks,
-                                              uint32_t bitsPerPixel);
+                                              uint32_t bitsPerPixel,
+                                              const SkCodec::Options& options);
 
     /*
      * Swizzle a row
      */
     SkSwizzler::ResultAlpha swizzle(void* dst, const uint8_t* SK_RESTRICT src);
 
     /**
      * Implement fill using a custom width.
      */
     void fill(const SkImageInfo& info, void* dst, size_t rowBytes, uint32_t colorOrIndex,
             SkCodec::ZeroInitialized zeroInit) override {
         const SkImageInfo fillInfo = info.makeWH(fDstWidth, info.height());
         SkSampler::Fill(fillInfo, dst, rowBytes, colorOrIndex, zeroInit);
     }
 
 private:
 
     /*
      * Row procedure used for swizzle
      */
-    typedef SkSwizzler::ResultAlpha (*RowProc)(
-            void* dstRow, const uint8_t* srcRow, int width,
+    typedef SkSwizzler::ResultAlpha (*RowProc)(void* dstRow, const uint8_t* srcRow, int width,
             SkMasks* masks, uint32_t startX, uint32_t sampleX);
 
-    /*
-     * Constructor for mask swizzler
-     */
-    SkMaskSwizzler(int width, SkMasks* masks, RowProc proc);
+    SkMaskSwizzler(SkMasks* masks, RowProc proc, int srcWidth, int srcOffset);
 
     int onSetSampleX(int) override;
 
     SkMasks*        fMasks;           // unowned
     const RowProc   fRowProc;
 
     // FIXME: Can this class share more with SkSwizzler? These variables are all the same.
     const int       fSrcWidth;        // Width of the source - i.e. before any sampling.
     int             fDstWidth;        // Width of dst, which may differ with sampling.
     int             fSampleX;
+    int             fSrcOffset;
     int             fX0;
 };
 
 #endif