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..5acc0b396c54d34abb922f80687104b54f85990a 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) {

[scroggo, 2015/07/21 16:48:56]

FWIW, we *could* check to see if fSubset matches the bounds, in which case it
means to decode the whole thing. This seems like a silly case to handle, so I
chose to take the fact that a subset was specified to mean that they wanted a
real subset. Let me know if you disagree and think we should handle that case.

[emmaleer, 2015/07/21 18:26:25]

I agree. I don't think we should support one specific size subset, and not other
size subsets. That could lead to confusion to the user why a certain subset
worked, and why a different one failed. 
I think it's best to fail for all size subsets.

+        // 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();