vo_vaapi: fix a crash on the init error handling path

No X display or libva can't be initialized -> crash.
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: prefer ffmpeg microdvd converter over internal one

We certainly don't want to maintain and improve the internal converter,
but we still need the internal one for Libav. (In the Libav case,
demux_subreader.c will be used to read the MicroDVD file.)