Move WebM writing support out of vpxenc.c.

This is mainly a clean up patchset. It moves the WebM writing support
out of vpxenc and into its own source file. Changes to tools_common and
vpxdec result from relocation of shared bits of code.

Change-Id: Iee55d3285f56e0a548f791094fb14c5ac5346a26
diff --git a/tools_common.h b/tools_common.h
index 9e56149..7dfd5ad 100644
--- a/tools_common.h
+++ b/tools_common.h
@@ -7,10 +7,24 @@
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
-#ifndef TOOLS_COMMON_H
-#define TOOLS_COMMON_H
+#ifndef TOOLS_COMMON_H_
+#define TOOLS_COMMON_H_
+
+#include <stdio.h>
+
+#define VP8_FOURCC (0x30385056)
+#define VP9_FOURCC (0x30395056)
+#define VP8_FOURCC_MASK (0x00385056)
+#define VP9_FOURCC_MASK (0x00395056)
 
 /* Sets a stdio stream into binary mode */
 FILE *set_binary_mode(FILE *stream);
 
-#endif
+void die(const char *fmt, ...);
+void fatal(const char *fmt, ...);
+void warn(const char *fmt, ...);
+
+/* The tool including this file must define usage_exit() */
+void usage_exit();
+
+#endif  // TOOLS_COMMON_H_