player: update playback position on seek

If the actual PTS is not known yet right after a seek, the "time-pos"
property will just return the seek target PTS. For this purpose, trigger
a change event to make the client API update the "time-pos" and related
properties. (MPV_EVENT_TICK triggers this update.)

player/playloop.c

@@ -324,6 +324,8 @@ static int mp_seek(MPContext *mpctx, struct seek_params seek,
     mpctx->start_timestamp = mp_time_sec();
     mpctx->sleeptime = 0;
 
+    mp_notify(mpctx, MPV_EVENT_TICK, NULL);
+
     return 0;
 }