Add scanline decoding to SkCodec.

src/codec/SkCodec.cpp

Index: src/codec/SkCodec.cpp
diff --git a/src/codec/SkCodec.cpp b/src/codec/SkCodec.cpp
index ec36bc75e94e00edd22b857b179a9033a95fe5c1..46c9bd7c97fe4c54c49d96136edacbdff20e227c 100644
--- a/src/codec/SkCodec.cpp
+++ b/src/codec/SkCodec.cpp
@@ -48,3 +48,23 @@ bool SkCodec::rewindIfNeeded() {
     fNeedsRewind = true;
     return !neededRewind || fStream->rewind();
 }
+
+SkScanlineDecoder* SkCodec::getScanlineDecoder(const SkImageInfo& dstInfo,
+                                               const SkIRect* origSubset) {
+    if (!rewindIfNeeded()) {
+        return NULL;
+    }
+    SkIRect correctedSubset;
+    if (NULL == origSubset) {
+        // Caller wants the whole image.
+        correctedSubset.setLTRB(0, 0, fInfo.width(), fInfo.height());
+    } else {
+        correctedSubset = *origSubset;
+        // Caller wants a subset. Make sure it intersects with the original
+        // image.
+        if (!correctedSubset.intersect(SkIRect::MakeSize(fInfo.dimensions()))) {
+            return NULL;
+        }
+    }
+    return this->onGetScanlineDecoder(dstInfo, correctedSubset);
+}