Respect mp4 track enable flag.

modules/demux/mp4/mp4.c

@@ -1989,6 +1989,8 @@ static void MP4_TrackCreate( demux_t *p_demux, mp4_track_t *p_track,
     /* do we launch this track by default ? */
     p_track->b_enable =
         ( ( p_tkhd->data.p_tkhd->i_flags&MP4_TRACK_ENABLED ) != 0 );
+    if( !p_track->b_enable )
+        p_track->fmt.i_priority = -1;
 
     p_track->i_track_ID = p_tkhd->data.p_tkhd->i_track_ID;
     p_track->i_width = p_tkhd->data.p_tkhd->i_width / 65536;