mkv: Flush the old cluster before writing a new one

This simplifies keeping track of cluster boundaries for e.g.
livestreaming of WebM.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/matroskaenc.c

@@ -1315,6 +1315,7 @@ static int mkv_write_packet(AVFormatContext *s, AVPacket *pkt)
         mkv->cluster_pos = 0;
         if (mkv->dyn_bc)
             mkv_flush_dynbuf(s);
+        avio_flush(s->pb);
     }
 
     // check if we have an audio packet cached