Add scanline decoding to SkCodec.

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index 245654dbfd3dbcb3c735b1f8ccdb44fe26433a8a..89b88d1b2fd0125bf54fefa808f74943c90dcb60 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -375,6 +375,11 @@ SkImageDecoder::Result SkPNGImageDecoder::onDecode(SkStream* sk_stream, SkBitmap
     const int number_passes = (interlaceType != PNG_INTERLACE_NONE) ?
                               png_set_interlace_handling(png_ptr) : 1;
 
+    if (number_passes > 1) {
+        // For simplified comparison, only do the ones that are not interlaced.
+        return kFailure;
+    }
+
     /* Optional call to gamma correct and add the background to the palette
     *  and update info structure.  REQUIRED if you are expecting libpng to
     *  update the palette for you (ie you selected such a transform above).