Add subsetting to SkScanlineDecoder

include/codec/SkCodec.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 SkCodec_DEFINED
 #define SkCodec_DEFINED
 
@@ skipping 150 matching lines @@
      */
     struct Options {
         Options()
             : fZeroInitialized(kNo_ZeroInitialized)
             , fSubset(NULL)
         {}
 
         ZeroInitialized fZeroInitialized;
         /**
          *  If not NULL, represents a subset of the original image to decode.
-         *
          *  Must be within the bounds returned by getInfo().
-         *
          *  If the EncodedFormat is kWEBP_SkEncodedFormat (the only one which
          *  currently supports subsets), the top and left values must be even.
+         *
+         *  In getPixels, we will attempt to decode the exact rectangular
+         *  subset specified by fSubset.
+         *
+         *  In a scanline decode, it does not make sense to specify a subset
+         *  top or subset height, since the client already controls which rows
+         *  to get and which rows to skip.  During scanline decodes, we will
+         *  require that the subset top be zero and the subset height be equal
+         *  to the full height.  We will, however, use the values of
+         *  subset left and subset width to decode partial scanlines on calls
+         *  to getScanlines().
          */
         SkIRect*        fSubset;
     };
 
     /**
      *  Decode into the given pixels, a block of memory of size at
      *  least (info.fHeight - 1) * rowBytes + (info.fWidth *
      *  bytesPerPixel)
      *
      *  Repeated calls to this function should give the same results,
@@ skipping 66 matching lines @@
      *  @param ctable A pointer to a color table.  When dstInfo.colorType() is
      *      kIndex8, this should be non-NULL and have enough storage for 256
      *      colors.  The color table will be populated after decoding the palette.
      *  @param ctableCount A pointer to the size of the color table.  When
      *      dstInfo.colorType() is kIndex8, this should be non-NULL.  It will
      *      be modified to the true size of the color table (<= 256) after
      *      decoding the palette.
      *  @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.
      */
     Result startScanlineDecode(const SkImageInfo& dstInfo);
 
     /**
      *  Write the next countLines scanlines into dst.
      *
@@ skipping 298 matching lines @@
      *  May create a sampler, if one is not currently being used. Otherwise, does
      *  not affect ownership.
      *
      *  Only valid during scanline decoding.
      */
     virtual SkSampler* getSampler(bool createIfNecessary) { return nullptr; }
 
     friend class SkScaledCodec;
 };
 #endif // SkCodec_DEFINED