lavc, lavf: move avformat static mutex from avcodec to avformat

It's completely absurd that libavcodec would care about libavformat
locking, but it was there because the lock manager was in libavcodec.

This is more stright forward. Changes ABI, but we don't require ABI
compatibility currently.

libavformat/utils.c

@@ -32,6 +32,7 @@
 #include "libavutil/opt.h"
 #include "libavutil/parseutils.h"
 #include "libavutil/pixdesc.h"
+#include "libavutil/thread.h"
 #include "libavutil/time.h"
 #include "libavutil/time_internal.h"
 #include "libavutil/timestamp.h"
@@ -55,6 +56,8 @@
 #include "libavutil/ffversion.h"
 const char av_format_ffversion[] = "FFmpeg version " FFMPEG_VERSION;
 
+static AVMutex avformat_mutex = AV_MUTEX_INITIALIZER;
+
 /**
  * @file
  * various utility functions for use within FFmpeg
@@ -77,6 +80,16 @@ const char *avformat_license(void)
     return LICENSE_PREFIX FFMPEG_LICENSE + sizeof(LICENSE_PREFIX) - 1;
 }
 
+int ff_lock_avformat(void)
+{
+    return ff_mutex_lock(&avformat_mutex) ? -1 : 0;
+}
+
+int ff_unlock_avformat(void)
+{
+    return ff_mutex_unlock(&avformat_mutex) ? -1 : 0;
+}
+
 #define RELATIVE_TS_BASE (INT64_MAX - (1LL<<48))
 
 static int is_relative(int64_t ts) {