If empty text is rendered, the bounding box is empty. Instead of
continuing with a bogus bounding box that would result in garbage
being rendered on screen, make the OSD image invisible.
This happened when playing demuxer SRT subtitles (e.g. SRT embedded in
MKV) with -no-ass at the moment a subtitle line disappeared.
Unrelated to this issue, fix libass API usage. Delete the event with
libass_flush_events(), instead of trying to reuse the previous event.
Based on a patch by uau.
wm4
b65424f5c2