Move all knowledge of X sampling into SkScaledCodec

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"
+#include "SkSampler.h"
 #include "SkSwizzler.h"
 #include "SkTypes.h"
 
 /*
  *
  * Used to swizzle images whose pixel components are extracted by bit masks
  * Currently only used by bmp
  *
  */
-class SkMaskSwizzler {
+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);
 
     /*
      * Swizzle a row
      */
     SkSwizzler::ResultAlpha swizzle(void* dst, const uint8_t* SK_RESTRICT src);
 
 private:
 
     /*
      * Row procedure used for swizzle
      */
     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(const SkImageInfo& info, SkMasks* masks, RowProc proc,
-            uint32_t sampleX);
+    SkMaskSwizzler(int width, SkMasks* masks, RowProc proc);
 
-    // Fields
-    const SkImageInfo& fDstInfo;
-    SkMasks*           fMasks;       // unowned
-    const RowProc      fRowProc;
-    const uint32_t     fSampleX;
-    const uint32_t     fStartX;
+    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             fX0;
 };
 
 #endif