Add the ability to decode a subset to SkCodec.

src/codec/SkJpegCodec.cpp

Index: src/codec/SkJpegCodec.cpp
diff --git a/src/codec/SkJpegCodec.cpp b/src/codec/SkJpegCodec.cpp
index 28e1e12bfc4ed7aa3272947a32662c03d72593c1..1f6bd0ea982f66cc7c5d19e1cd7a9c5367e0e17e 100644
--- a/src/codec/SkJpegCodec.cpp
+++ b/src/codec/SkJpegCodec.cpp
@@ -308,6 +308,11 @@ SkCodec::Result SkJpegCodec::onGetPixels(const SkImageInfo& dstInfo,
         return fDecoderMgr->returnFailure("could not rewind stream", kCouldNotRewind);
     }
 
+    if (!options.fSubset.isEmpty()) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
+
     // Get a pointer to the decompress info since we will use it quite frequently
     jpeg_decompress_struct* dinfo = fDecoderMgr->dinfo();