Add subsetting to SkScanlineDecoder

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index 1b9cb0f5d7e3afb73b0fda2dde8a593e55058ed4..5938ded30a148b05ed2b7226d066bc888bfd1af4 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -173,6 +173,16 @@ public:
          *
          *  If the EncodedFormat is kWEBP_SkEncodedFormat (the only one which
          *  currently supports subsets), the top and left values must be even.
+         *
+         *  The above description applies to getPixels() but the usage is
+         *  slightly different when we want to startScanlineDecode().  It
+         *  does not make a lot of sense to specify a fSubsetTop or
+         *  fSubsetHeight when scanline decoding, since the client already
+         *  controls which rows to get and which rows to skip.  During scanline
+         *  decodes, we will require that fSubsetTop be zero and fSubsetHeight
+         *  be the full height.  We will, however, use the values of
+         *  fSubsetLeft and fSubsetWidth to decode partial scanlines on calls
+         *  to getScanlines().
          */
         SkIRect*        fSubset;
     };
@@ -259,11 +269,11 @@ public:
      *  @return Enum representing success or reason for failure.
      */
     Result startScanlineDecode(const SkImageInfo& dstInfo, const SkCodec::Options* options,
-                 SkPMColor ctable[], int* ctableCount);
+            SkPMColor ctable[], int* ctableCount);
 
     /**
      *  Simplified version of startScanlineDecode() that asserts that info is NOT
-     *  kIndex8_SkColorType and uses the default Options.
+     *  kIndex8_SkColorType and uses the default options.
      */
     Result startScanlineDecode(const SkImageInfo& dstInfo);