sd_ass.c: set event->Style to fix --ass-styles
Set the "Style" attribute for subtitle events created in sd_ass to
match the "default_style" attribute of the track. This is required to
make --ass-styles work with recent libass versions (otherwise the
event would use style 0, which is a fallback style added by libass
now).
Commit
378ada847c8eb3a641392691b1a2317f8fc214e0 ("sub/ass: use default
style, not first style unconditionally") fixed the same issue for
external subtitles converted with mp_ass_read_subdata(); this commit
fixes it for muxed ones.