Add the ability to decode a subset to SkCodec.

src/codec/SkCodec_libgif.cpp

Index: src/codec/SkCodec_libgif.cpp
diff --git a/src/codec/SkCodec_libgif.cpp b/src/codec/SkCodec_libgif.cpp
index fb578f2bea6de8b8efded798b43500a90a64af4a..9b15151f1066d876500cd980caad2f936af2980f 100644
--- a/src/codec/SkCodec_libgif.cpp
+++ b/src/codec/SkCodec_libgif.cpp
@@ -257,6 +257,10 @@ SkCodec::Result SkGifCodec::onGetPixels(const SkImageInfo& dstInfo,
     }
 
     // Check for valid input parameters
+    if (opts.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
     if (dstInfo.dimensions() != this->getInfo().dimensions()) {
         return gif_error("Scaling not supported.\n", kInvalidScale);
     }