vd_lavc: improve a comment
Rename sub.c/.h to osd.c/.h

This was way too misleading. osd.c merely calls the subtitle renderers,
instead of actually dealing with subtitles.
osd: remove mp_osd_res.video_par field

This is not needed anymore, because we decided that the PAR of the
decoded video matters, and not the PAR of the filtered video that
arrives at the VO.
player: close demuxer before stream

Demuxer might access stream even when closing. For now, this is not
a real problem (because it didn't actually happen), but it's cleaner.