Make SkPngCodec decode progressively.

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 "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.
+     */
+    bool rowNeeded(int row) const {
+        return row % 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.
      *
      * @param dst
      * The destination row to fill from.
      *
@@ skipping 14 matching lines @@
      */
     static void Fill(const SkImageInfo& info, void* dst, size_t rowBytes,
             uint64_t colorOrIndex, SkCodec::ZeroInitialized zeroInit);
 
     /**
      * Allow subclasses to implement unique versions of fill().
      */
     virtual void fill(const SkImageInfo& info, void* dst, size_t rowBytes,
             uint64_t colorOrIndex, SkCodec::ZeroInitialized zeroInit) {}
 
+    SkSampler()
+        : fSampleY(1)
+    {}
+
     virtual ~SkSampler() {}
 private:
+    int fSampleY;
 
     virtual int onSetSampleX(int) = 0;
 };
 
 #endif // SkSampler_DEFINED