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_libpng.h"
@@ skipping 30 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,
+                                               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);
+}