video/out: change aspects of OSD handling

Let the VOs draw the OSD on their own, instead of making OSD drawing a
separate VO driver call. Further, let it be the VOs responsibility to
request subtitles with the correct PTS. We also basically allow the VO
to request OSD/subtitles at any time.

OSX changes untested.

sub/osd_state.h

@@ -52,8 +52,6 @@ struct osd_state {
 
     struct osd_object *objs[MAX_OSD_PARTS];
 
-    double vo_pts;
-
     bool render_subs_in_filter;
 
     bool want_redraw;