Add the ability to decode a subset to SkCodec.

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index cc635e012ca82bd2c11839ab10fe51ab5f9625a7..8a4d0142aae39d9b8de5927edabb04d02129b5b8 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -128,9 +128,16 @@ public:
      */
     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;
     };
 
     /**