Add the ability to decode a subset to SkCodec.

src/codec/SkCodec_libpng.cpp

Index: src/codec/SkCodec_libpng.cpp
diff --git a/src/codec/SkCodec_libpng.cpp b/src/codec/SkCodec_libpng.cpp
index 699ab398d3a36df70cfbbb1c24c96886047d7f3c..8278856c22e41eb849f40d90c86c431fb3f1bf89 100644
--- a/src/codec/SkCodec_libpng.cpp
+++ b/src/codec/SkCodec_libpng.cpp
@@ -517,6 +517,10 @@ SkCodec::Result SkPngCodec::onGetPixels(const SkImageInfo& requestedInfo, void*
     if (!conversion_possible(requestedInfo, this->getInfo())) {
         return kInvalidConversion;
     }
+    if (options.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
     if (requestedInfo.dimensions() != this->getInfo().dimensions()) {
         return kInvalidScale;
     }