Make SkCodec support peek() and read()

src/codec/SkJpegCodec.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 "SkJpegCodec.h"
 #include "SkJpegDecoderMgr.h"
 #include "SkJpegUtility_codec.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkStream.h"
 #include "SkTemplates.h"
 #include "SkTypes.h"
 
 // stdio is needed for libjpeg-turbo
 #include <stdio.h>
 
 extern "C" {
     #include "jerror.h"
     #include "jpeglib.h"
 }
 
-bool SkJpegCodec::IsJpeg(SkStream* stream) {
+bool SkJpegCodec::IsJpeg(const char* buffer, size_t bytesRead) {
     static const uint8_t jpegSig[] = { 0xFF, 0xD8, 0xFF };
-    char buffer[sizeof(jpegSig)];
-    return stream->read(buffer, sizeof(jpegSig)) == sizeof(jpegSig) &&
-            !memcmp(buffer, jpegSig, sizeof(jpegSig));
+    return bytesRead >= 3 && !memcmp(buffer, jpegSig, sizeof(jpegSig));
 }
 
 bool SkJpegCodec::ReadHeader(SkStream* stream, SkCodec** codecOut,
         JpegDecoderMgr** decoderMgrOut) {
 
     // Create a JpegDecoderMgr to own all of the decompress information
     SkAutoTDelete<JpegDecoderMgr> decoderMgr(new JpegDecoderMgr(stream));
 
     // libjpeg errors will be caught and reported here
     if (setjmp(decoderMgr->getJmpBuf())) {
@@ skipping 406 matching lines @@
 #endif
 
 bool SkJpegCodec::onSkipScanlines(int count) {
     // Set the jump location for libjpeg errors
     if (setjmp(fDecoderMgr->getJmpBuf())) {
         return fDecoderMgr->returnFalse("setjmp");
     }
 
     return (uint32_t) count == jpeg_skip_scanlines(fDecoderMgr->dinfo(), count);
 }