Add the ability to decode a subset to SkCodec.

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 110 matching lines @@
          *  This is the default. It will be used if no Options struct is used.
          */
         kNo_ZeroInitialized,
     };
 
     /**
      *  Additional options to pass to getPixels.
      */
     struct Options {
         Options()
-            : fZeroInitialized(kNo_ZeroInitialized) {}
+            : fZeroInitialized(kNo_ZeroInitialized)
+        {
+            fSubset.setEmpty();
+        }
 
         ZeroInitialized fZeroInitialized;
+        /**
+         *  If not empty, represents a subset of the original image to decode.
+         */
+        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,
      *  allowing the PixelRef to be immutable.
      *
@@ skipping 137 matching lines @@
     SkStream* stream() {
         return fStream.get();
     }
 
 private:
     const SkImageInfo       fInfo;
     SkAutoTDelete<SkStream> fStream;
     bool                    fNeedsRewind;
 };
 #endif // SkCodec_DEFINED