Add the ability to decode a subset to SkCodec.

dm/DM.cpp

Index: dm/DM.cpp
diff --git a/dm/DM.cpp b/dm/DM.cpp
index dd072dcec6d86fbf87fa1970175b5ce645f79024..d472be69d93374cb0e9a5d81085e534642b2182a 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -257,6 +257,11 @@ static void push_codec_srcs(Path path) {
                 CodecSrc::kGetFromCanvas_DstColorType, scale));
         push_src("image", "stripe", new CodecSrc(path, CodecSrc::kStripe_Mode,
                 CodecSrc::kGetFromCanvas_DstColorType, scale));
+        // Note: The only codec which supports subsets natively is SkWebpCodec, which will never
+        // report kIndex_8 or kGray_8, so there is no need to test kSubset_mode with those color
+        // types specifically requested.
+        push_src("image", "codec_subset", new CodecSrc(path, CodecSrc::kSubset_Mode,
+                CodecSrc::kGetFromCanvas_DstColorType, scale));
     }
 }