add shell for libwebpextras

meant to contain additional utility functions useful in processing webp
input/output.

Change-Id: I014ae6b917d62e826aa23a3bbe99aac4462a97c2

src/webp/extras.h

@@ -0,0 +1,34 @@
+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the COPYING file in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS. All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+// -----------------------------------------------------------------------------
+//
+
+#ifndef WEBP_WEBP_EXTRAS_H_
+#define WEBP_WEBP_EXTRAS_H_
+
+#include "./types.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define WEBP_EXTRAS_ABI_VERSION 0x0000    // MAJOR(8b) + MINOR(8b)
+
+//------------------------------------------------------------------------------
+
+// Returns the version number of the extras library, packed in hexadecimal using
+// 8bits for each of major/minor/revision. E.g: v2.5.7 is 0x020507.
+WEBP_EXTERN(int) WebPGetExtrasVersion(void);
+
+//------------------------------------------------------------------------------
+
+#ifdef __cplusplus
+}    // extern "C"
+#endif
+
+#endif  /* WEBP_WEBP_EXTRAS_H_ */