Make SkPngCodec decode progressively.

src/codec/SkSampler.h

Index: src/codec/SkSampler.h
diff --git a/src/codec/SkSampler.h b/src/codec/SkSampler.h
index b233c32be22942577fb4502c7e722662417c2b82..00015585a11dda1fcf74e9ecc02ad9bec7f58662 100644
--- a/src/codec/SkSampler.h
+++ b/src/codec/SkSampler.h
@@ -21,6 +21,30 @@ public:
     }
 
     /**
+     *  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;

[msarett, 2016/09/14 18:16:05]

I actually think for this to work properly, it depends on the subset being
specified with tight bounds...

Ex: We always want to keep row 0, when the subset is specified this way.

[scroggo_chromium, 2016/09/14 22:01:41]

Agreed. I think the comment describes that? ("starting with the first row used
in the output")

+    }
+
+    /**
      * Fill the remainder of the destination with a single color
      *
      * @param info
@@ -55,8 +79,13 @@ public:
     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;
 };