Currently by default mpv sets --all-subs when calling yt-dlp for JSON
info. --all-subs is an option that simply expands to --sub-langs all
--write-subs. --write-subs is special that without it yt-dlp will no
longer extract any subtitle info.
mpv's yt-dlp hook has a logic that if user specify any sub-langs in
--ytdl-raw-options, it no longer uses --all-subs. Unfortunately, this
means unaware users will not get any subtitle any more until they study
the mpv internal code and add their own --write-subs.
This PR always specifies --write-srt, an alias of --write-subs, in the
hook, so users don't have to. The alias is used to maintain
compatibility with youtube-dl.
Fixes issue #14818
Crend King
c8860fd816