Actually add flag (somehow got lost in my previous patchset?), fixes broken

r22806.



git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22807 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/avformat.h

@@ -119,6 +119,7 @@ struct AVFormatContext;
 #define AV_METADATA_IGNORE_SUFFIX   2
 #define AV_METADATA_DONT_STRDUP_KEY 4
 #define AV_METADATA_DONT_STRDUP_VAL 8
+#define AV_METADATA_DONT_OVERWRITE 16   ///< Don't overwrite existing tags.
 
 typedef struct {
     char *key;