subs: always use sub decoder framework for libass rendering

subs: always use sub decoder framework for libass rendering

Remove subtitle selection code setting osd->ass_track directly and
vf_ass/vf_vo code rendering the track directly with libass. Instead,
do track selection and rendering with dec_sub.c functions.

Before, mpctx->set_of_ass_tracks[] contained bare libass tracks
generated from external subtitle files. For use with dec_sub.c, it now
contains struct sh_sub instances with decoder already initialized.

This commit breaks the sub_step command ('g' and 'y' keys) for
libass-rendered subtitles. It could be fixed, but it's so useless -
especially as with the existing implementation there's no practical
way to get subtitle delay back to normal after using it - that I
didn't bother.