Fix SkSwizzler bug

src/codec/SkMaskSwizzler.cpp

 /*
  * 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 "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkMaskSwizzler.h"
@@ skipping 349 matching lines @@
     }
 
     return new SkMaskSwizzler(masks, proc, srcOffset, srcWidth);
 }
 
 /*
  *
  * Constructor for mask swizzler
  *
  */
-SkMaskSwizzler::SkMaskSwizzler(SkMasks* masks, RowProc proc, int srcOffset, int srcWidth)
+SkMaskSwizzler::SkMaskSwizzler(SkMasks* masks, RowProc proc, int srcOffset, int subsetWidth)
     : fMasks(masks)
     , fRowProc(proc)
-    , fSrcWidth(srcWidth)
-    , fDstWidth(srcWidth)
+    , fSubsetWidth(subsetWidth)
+    , fDstWidth(subsetWidth)
     , fSampleX(1)
     , fSrcOffset(srcOffset)
     , fX0(srcOffset)
 {}
 
 int SkMaskSwizzler::onSetSampleX(int sampleX) {
     // FIXME: Share this function with SkSwizzler?
     SkASSERT(sampleX > 0); // Surely there is an upper limit? Should there be
                            // way to report failure?
     fSampleX = sampleX;
     fX0 = get_start_coord(sampleX) + fSrcOffset;
-    fDstWidth = get_scaled_dimension(fSrcWidth, sampleX);
+    fDstWidth = get_scaled_dimension(fSubsetWidth, sampleX);
 
-    // check that fX0 is less than original width
-    SkASSERT(fX0 >= 0 && fX0 < fSrcWidth);
+    // check that fX0 is valid
+    SkASSERT(fX0 >= 0);
     return fDstWidth;
 }
 
 /*
  *
  * Swizzle the specified row
  *
  */
 SkSwizzler::ResultAlpha SkMaskSwizzler::swizzle(void* dst, const uint8_t* SK_RESTRICT src) {
     SkASSERT(nullptr != dst && nullptr != src);
     return fRowProc(dst, src, fDstWidth, fMasks, fX0, fSampleX);
 }