Add the ability to decode a subset to SkCodec.

src/codec/SkCodec_libbmp.cpp

Index: src/codec/SkCodec_libbmp.cpp
diff --git a/src/codec/SkCodec_libbmp.cpp b/src/codec/SkCodec_libbmp.cpp
index 3ac4b0b8ab1828cbac4fe27bbdd373e9be4609e1..bd5d2ca7c557fa7c4157428b8e5201cde2320254 100644
--- a/src/codec/SkCodec_libbmp.cpp
+++ b/src/codec/SkCodec_libbmp.cpp
@@ -585,6 +585,10 @@ SkCodec::Result SkBmpCodec::onGetPixels(const SkImageInfo& dstInfo,
             return kCouldNotRewind;
         }
     }
+    if (opts.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
     if (dstInfo.dimensions() != this->getInfo().dimensions()) {
         SkCodecPrintf("Error: scaling not supported.\n");
         return kInvalidScale;