Add the ability to decode a subset to SkCodec.

src/codec/SkCodec_libico.cpp

Index: src/codec/SkCodec_libico.cpp
diff --git a/src/codec/SkCodec_libico.cpp b/src/codec/SkCodec_libico.cpp
index 97404afed056bd701422a4a1b6f8fcb07365d384..7df4879442ed1931d987609e1bddac69db905d5e 100644
--- a/src/codec/SkCodec_libico.cpp
+++ b/src/codec/SkCodec_libico.cpp
@@ -229,6 +229,10 @@ SkCodec::Result SkIcoCodec::onGetPixels(const SkImageInfo& dstInfo,
                                         void* dst, size_t dstRowBytes,
                                         const Options& opts, SkPMColor* ct,
                                         int* ptr) {
+    if (opts.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
     // We return invalid scale if there is no candidate image with matching
     // dimensions.
     Result result = kInvalidScale;