Add the ability to decode a subset to SkCodec.

src/codec/SkCodec_libgif.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_libgif.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
@@ skipping 239 matching lines @@
         GifFileType* gifOut = NULL;
         if (!ReadHeader(this->stream(), NULL, &gifOut)) {
             return kCouldNotRewind;
         } else {
             SkASSERT(NULL != gifOut);
             fGif.reset(gifOut);
         }
     }
 
     // Check for valid input parameters
+    if (opts.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
     if (dstInfo.dimensions() != this->getInfo().dimensions()) {
         return gif_error("Scaling not supported.\n", kInvalidScale);
     }
     if (!conversion_possible(dstInfo, this->getInfo())) {
         return gif_error("Cannot convert input type to output type.\n",
                 kInvalidConversion);
     }
 
     // Use this as a container to hold information about any gif extension
     // blocks.  This generally stores transparency and animation instructions.
@@ skipping 291 matching lines @@
                 // giflib returns an error code if the record type is not known.
                 // We should catch this error immediately.
                 SkASSERT(false);
                 break;
         }
     } while (TERMINATE_RECORD_TYPE != recordType);
 
     return gif_error("Could not find any images to decode in gif file.\n",
             kInvalidInput);
 }