Make SkCodec support peek() and read()

src/codec/SkCodec_libgif.h

 /*
  * 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 "SkColorTable.h"
 #include "SkImageInfo.h"
 #include "SkSwizzler.h"
 
 #include "gif_lib.h"
 
 /*
  *
  * This class implements the decoding for gif images
  *
  */
 class SkGifCodec : public SkCodec {
 public:
-
-    /*
-     * Checks the start of the stream to see if the image is a gif
-     */
-    static bool IsGif(SkStream*);
+    static bool IsGif(const void*, size_t);
 
     /*
      * Assumes IsGif was called and returned true
      * Creates a gif decoder
      * Reads enough of the stream to determine the image format
      */
     static SkCodec* NewFromStream(SkStream*);
 
 protected:
 
@@ skipping 161 matching lines @@
     SkAutoTDeleteArray<uint8_t>             fSrcBuffer;
     const SkIRect                           fFrameRect;
     const uint32_t                          fTransIndex;
     uint32_t                                fFillIndex;
     const bool                              fFrameIsSubset;
     SkAutoTDelete<SkSwizzler>               fSwizzler;
     SkAutoTUnref<SkColorTable>              fColorTable;
 
     typedef SkCodec INHERITED;
 };