Add the ability to decode a subset to SkCodec.

src/codec/SkCodec_libpng.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_libpng.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
@@ skipping 499 matching lines @@
             return false;
     }
 }
 
 SkCodec::Result SkPngCodec::onGetPixels(const SkImageInfo& requestedInfo, void* dst,
                                         size_t rowBytes, const Options& options,
                                         SkPMColor ctable[], int* ctableCount) {
     if (!conversion_possible(requestedInfo, this->getInfo())) {
         return kInvalidConversion;
     }
+    if (options.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
     if (requestedInfo.dimensions() != this->getInfo().dimensions()) {
         return kInvalidScale;
     }
     if (!this->handleRewind()) {
         return kCouldNotRewind;
     }
 
     // Note that ctable and ctableCount may be modified if there is a color table
     const Result result = this->initializeSwizzler(requestedInfo, dst, rowBytes,
                                                    options, ctable, ctableCount);
@@ skipping 230 matching lines @@
 
     SkASSERT(codec->fNumberPasses != INVALID_NUMBER_PASSES);
     if (codec->fNumberPasses > 1) {
         // interlaced image
         return SkNEW_ARGS(SkPngInterlacedScanlineDecoder, (dstInfo, codec.detach()));
     }
 
     return SkNEW_ARGS(SkPngScanlineDecoder, (dstInfo, codec.detach()));
 }