Add the ability to decode a subset to SkCodec.

src/codec/SkCodec_wbmp.cpp

Index: src/codec/SkCodec_wbmp.cpp
diff --git a/src/codec/SkCodec_wbmp.cpp b/src/codec/SkCodec_wbmp.cpp
index 9709a689b12b5ab9e1dee519e27282a37d7e0145..35ac808219c625ad1bacf045d6414fb4b8e40b76 100644
--- a/src/codec/SkCodec_wbmp.cpp
+++ b/src/codec/SkCodec_wbmp.cpp
@@ -103,7 +103,7 @@ SkEncodedFormat SkWbmpCodec::onGetEncodedFormat() const {
 SkCodec::Result SkWbmpCodec::onGetPixels(const SkImageInfo& info,
                                          void* pixels,
                                          size_t rowBytes,
-                                         const Options&,
+                                         const Options& options,
                                          SkPMColor ctable[],
                                          int* ctableCount) {
     SkCodec::RewindState rewindState = this->rewindIfNeeded();
@@ -112,6 +112,10 @@ SkCodec::Result SkWbmpCodec::onGetPixels(const SkImageInfo& info,
     } else if (rewindState == kRewound_RewindState) {
         (void)read_header(this->stream(), NULL);
     }
+    if (options.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
     if (info.dimensions() != this->getInfo().dimensions()) {
         return kInvalidScale;
     }