Use fActualProc to choose fFastProc or fSlowProc in SkSwizzler

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 144 matching lines @@
                             const uint8_t* SK_RESTRICT src,
                             int dstWidth, int bpp, int deltaSrc, int offset,
                             const SkPMColor ctable[]);
 
     template <RowProc Proc>
     static void SkipLeading8888ZerosThen(void* SK_RESTRICT dstRow,
                                          const uint8_t* SK_RESTRICT src,
                                          int dstWidth, int bpp, int deltaSrc, int offset,
                                          const SkPMColor ctable[]);
 
-    // May be NULL.  We will not always be able to used an optimized function.
-    RowProc             fFastProc;
-    // Always non-NULL.  We use this if fFastProc is NULL.
-    const RowProc       fProc;
+    // May be NULL.  We have not implemented optimized functions for all supported transforms.
+    const RowProc       fFastProc;
+    // Always non-NULL.  Supports sampling.
+    const RowProc       fSlowProc;
+    // The actual RowProc we are using.  This depends on if fFastProc is non-NULL and
+    // whether or not we are sampling.
+    RowProc             fActualProc;
+
     const SkPMColor*    fColorTable;      // Unowned pointer
 
     // Subset Swizzles
     // There are two types of subset swizzles that we support.  We do not
     // support both at the same time.
     // TODO: If we want to support partial scanlines for gifs (which may
     //       use frame subsets), we will need to support both subsetting
     //       modes at the same time.
     // (1) Partial Scanlines
     //         The client only wants to write a subset of the source pixels
@@ skipping 73 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