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 73e11c986e2949981a4f215678582fdfce060e8c..1a75d482fb7495e6b9c94ea37e5fd8a39fe2be73 100644
--- a/src/codec/SkSampler.h
+++ b/src/codec/SkSampler.h
@@ -21,6 +21,28 @@ 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 Zero-based index into the subset.
+     */
+    bool rowNeeded(int row) const {
+        return row % fSampleY == 0;

[msarett, 2016/05/24 13:14:07]

Is this correct?  I think we don't start at row 0...
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/s...

Maybe this would be useful?
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/s...

[scroggo, 2016/05/24 14:24:50]

I believe it's used correctly by SkPngCodec, but it is unclear. Note how we
actually pass fLinesDecoded, before incrementing it, so we start with zero, even
though we may not be decoding row 0.
We could use that method, although from the current call site, I don't know the
scaledDim. I could plumb it in though. I think rowNeeded would work, but maybe
needs some better documentation? PTAL at the new comment. It might be a tad
circular though...

I guess the ugly part is that once we start supporting bottom up BMPs through
the new path we won't be able to just use fLinesDecoded; we'll need to something
more like is_coord_necessary.

[msarett, 2016/05/24 14:28:33]

Gotcha thanks.
New comment looks good.
It's fine with me if we figure this out when we get here.

+    }
+
+    /**
      * Fill the remainder of the destination with a single color
      *
      * @param info
@@ -54,8 +76,13 @@ public:
     virtual void fill(const SkImageInfo& info, void* dst, size_t rowBytes,
             uint32_t colorOrIndex, SkCodec::ZeroInitialized zeroInit) {}
 
+    SkSampler()
+        : fSampleY(1)
+    {}
+
     virtual ~SkSampler() {}
 private:
+    int fSampleY;
 
     virtual int onSetSampleX(int) = 0;
 };