Incremental decode: only use subset for subsetting

src/codec/SkSampler.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 SkSampler_DEFINED
 #define SkSampler_DEFINED
 
 #include "SkCodec.h"
+#include "SkCodecPriv.h"
 #include "SkTypes.h"
 
 class SkSampler : public SkNoncopyable {
 public:
     /**
      *  Update the sampler to sample every sampleX'th pixel. Returns the
      *  width after sampling.
      */
     int setSampleX(int sampleX) {
         return this->onSetSampleX(sampleX);
     }
 
     /**
      *  Update the sampler to sample every sampleY'th row.
      */
     void setSampleY(int sampleY) {
         fSampleY = sampleY;
     }
 
     /**
      *  Retrieve the value set for sampleY.
      */
     int sampleY() const {
         return fSampleY;
     }
 
     /**
      *  Based on fSampleY, return whether this row belongs in the output.
      *
-     *  @param row Row of the image, starting with the first row used in the
-     *      output.
+     *  @param row Row of the image, starting with the first row in the subset.
      */
     bool rowNeeded(int row) const {
-        return row % fSampleY == 0;
+        return (row + get_start_coord(fSampleY)) % fSampleY == 0;
     }
 
     /**
      * Fill the remainder of the destination with a single color
      *
      * @param info
      * Contains the color type of the rows to fill.
      * Contains the width of the destination rows to fill
      * Contains the number of rows that we need to fill.
      *
@@ skipping 29 matching lines @@
     {}
 
     virtual ~SkSampler() {}
 private:
     int fSampleY;
 
     virtual int onSetSampleX(int) = 0;
 };
 
 #endif // SkSampler_DEFINED