handle_osd_redraw is called while we're in the middle of the cleanup portion
of play_current_file (via kill_demuxers_reentrant and mp_idle). This happens
after we've torn down the subtitle track, so we end up requesting a redraw
from the vo, which results in a 1-frame flash of the video without subs
before we either quit or switch to the next file (or the force-window state).
This detects that condition and skips the redraw, without interfering with
drawing the osd during idle (eg for pseudo-gui mode).
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