Add scanline decoding to SkCodec.

src/codec/SkCodec.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkData.h"
 #include "SkCodec_libbmp.h"
@@ skipping 41 matching lines @@
     , fNeedsRewind(false)
 {}
 
 bool SkCodec::rewindIfNeeded() {
     // Store the value of fNeedsRewind so we can update it. Next read will
     // require a rewind.
     const bool neededRewind = fNeedsRewind;
     fNeedsRewind = true;
     return !neededRewind || fStream->rewind();
 }
+
+SkScanlineDecoder* SkCodec::getScanlineDecoder(const SkImageInfo& dstInfo) {
+    fScanlineDecoder.reset(NULL);
+    if (!rewindIfNeeded()) {
+        return NULL;
+    }
+    fScanlineDecoder.reset(this->onGetScanlineDecoder(dstInfo));
+    return fScanlineDecoder.get();
+}