sd_ass: remove subreader use, support plaintext markup

sd_ass: remove subreader use, support plaintext markup

Originally, when rendering plaintext subs with libass, the subtitles
were first converted to the "struct subtitle" form with sub_add_text()
and then from that to libass events. Change sd_ass to convert the
subtitles directly to libass events without using the old sub
machinery. The new conversion at least fixes some timing issues. Also
use the markup support added in the previous commit, so that
HTML-style markup is also supported in "plaintext" subs rendered with
libass.