Make SkCodec support peek() and read()

src/codec/SkBmpCodec.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 "SkBmpCodec.h"
 #include "SkBmpMaskCodec.h"
 #include "SkBmpRLECodec.h"
@@ skipping 38 matching lines @@
 enum BmpInputFormat {
     kStandard_BmpInputFormat,
     kRLE_BmpInputFormat,
     kBitMask_BmpInputFormat,
     kUnknown_BmpInputFormat
 };
 
 /*
  * Checks the start of the stream to see if the image is a bitmap
  */
-bool SkBmpCodec::IsBmp(SkStream* stream) {
+bool SkBmpCodec::IsBmp(const void* buffer, size_t bytesRead) {
     // TODO: Support "IC", "PT", "CI", "CP", "BA"
     const char bmpSig[] = { 'B', 'M' };
-    char buffer[sizeof(bmpSig)];
-    return stream->read(buffer, sizeof(bmpSig)) == sizeof(bmpSig) &&
-            !memcmp(buffer, bmpSig, sizeof(bmpSig));
+    return bytesRead >= sizeof(bmpSig) && !memcmp(buffer, bmpSig, sizeof(bmpSig));
 }
 
 /*
  * Assumes IsBmp was called and returned true
  * Creates a bmp decoder
  * Reads enough of the stream to determine the image format
  */
 SkCodec* SkBmpCodec::NewFromStream(SkStream* stream) {
     return SkBmpCodec::NewFromStream(stream, false);
 }
@@ skipping 485 matching lines @@
     return prepareToDecode(dstInfo, options, inputColorPtr, inputColorCount);
 }
 
 int SkBmpCodec::onGetScanlines(void* dst, int count, size_t rowBytes) {
     // Create a new image info representing the portion of the image to decode
     SkImageInfo rowInfo = this->dstInfo().makeWH(this->dstInfo().width(), count);
 
     // Decode the requested rows
     return this->decodeRows(rowInfo, dst, rowBytes, this->options());
 }