player: show subtitles on VO if --force-window is used

If a VO is created, but no video is playing (i.e. --force-window is
used), then until now no subtitles were shown. This is because VO
subtitle display normally depends on video frame timing. If there are no
video frames, there can be no subtitles.

Change this and add some code to handle this situation specifically. Set
a subtitle PTS manually and request VO redrawing manually, which gets
the subtitles rendered somehow.

This is kind of shaky. The subtitles are essentially sampled at
arbitrary times (such as when new audio data is decoded and pushed to
the AO, or on user interaction). To make a it slightly more consistent,
force a completely arbitrary minimum FPS of 10.

Other solutions (such as creating fake video) would be more intrusive or
would require VO-level API changes.

Fixes #3684.

sub/osd_state.h

@@ -67,6 +67,7 @@ struct osd_state {
     struct osd_object *objs[MAX_OSD_PARTS];
 
     bool render_subs_in_filter;
+    double force_video_pts;
 
     bool want_redraw;
     bool want_redraw_notification;