Make SkCodec support peek() and read()

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"
 #include "SkColorTable.h"
 #include "SkStream.h"
 #include "SkSwizzler.h"
 #include "SkUtils.h"
 
 /*
  * Checks the start of the stream to see if the image is a gif
  */
-bool SkGifCodec::IsGif(SkStream* stream) {
-    char buf[GIF_STAMP_LEN];
-    if (stream->read(buf, GIF_STAMP_LEN) == GIF_STAMP_LEN) {
+bool SkGifCodec::IsGif(const char* buf, size_t bytesRead) {
+    if (bytesRead >= GIF_STAMP_LEN) {
         if (memcmp(GIF_STAMP,   buf, GIF_STAMP_LEN) == 0 ||
             memcmp(GIF87_STAMP, buf, GIF_STAMP_LEN) == 0 ||
             memcmp(GIF89_STAMP, buf, GIF_STAMP_LEN) == 0)
         {
             return true;
         }
     }
     return false;
 }
 
@@ skipping 550 matching lines @@
 int SkGifCodec::onOutputScanline(int inputScanline) const {
     if (fGif->Image.Interlace) {
         if (inputScanline < fFrameRect.top() || inputScanline >= fFrameRect.bottom()) {
             return inputScanline;
         }
         return get_output_row_interlaced(inputScanline - fFrameRect.top(), fFrameRect.height()) +
                 fFrameRect.top();
     }
     return inputScanline;
 }