Make SkCodec support peek() and read()

src/codec/SkCodec_libpng.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_libpng.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
@@ skipping 188 matching lines @@
     }
 
     fColorTable.reset(new SkColorTable(colorStorage, colorCount));
     return true;
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 // Creation
 ///////////////////////////////////////////////////////////////////////////////
 
-#define PNG_BYTES_TO_CHECK 4
-
-bool SkPngCodec::IsPng(SkStream* stream) {
-    char buf[PNG_BYTES_TO_CHECK];
-    if (stream->read(buf, PNG_BYTES_TO_CHECK) != PNG_BYTES_TO_CHECK) {
-        return false;
-    }
-    if (png_sig_cmp((png_bytep) buf, (png_size_t)0, PNG_BYTES_TO_CHECK)) {
-        return false;
-    }
-    return true;
+bool SkPngCodec::IsPng(const char* buf, size_t bytesRead) {
+    return !png_sig_cmp((png_bytep) buf, (png_size_t)0, bytesRead);
 }
 
 // Reads the header and initializes the output fields, if not NULL.
 //
 // @param stream Input data. Will be read to get enough information to properly
 //      setup the codec.
 // @param chunkReader SkPngChunkReader, for reading unknown chunks. May be NULL.
 //      If not NULL, png_ptr will hold an *unowned* pointer to it. The caller is
 //      expected to continue to own it for the lifetime of the png_ptr.
 // @param png_ptrp Optional output variable. If non-NULL, will be set to a new
@@ skipping 636 matching lines @@
     }
 
     if (1 == numberPasses) {
         return new SkPngScanlineDecoder(imageInfo, streamDeleter.detach(), chunkReader,
                                         png_ptr, info_ptr, bitDepth);
     }
 
     return new SkPngInterlacedScanlineDecoder(imageInfo, streamDeleter.detach(), chunkReader,
                                               png_ptr, info_ptr, bitDepth, numberPasses);
 }