Now --ass-use-margins doesn't apply to normal subtitles anymore. This is
probably the inverse from the mpv behavior users expected so far, and
thus a breaking change, so rename the option, that the user at least has
a chance to lookup the option and decide whether the new behavior is
wanted or not.
The basic idea here is:
- plain text subtitles should have a certain useful defalt behavior,
like actually using margins
- ASS subtitles should never be broken by default
- ASS subtitles should look and behave like plaintext subtitles if
the --ass-style-override=force option is used
This also subtly changes --sub-scale-with-window and adds the --ass-
scale-with-window option. Since this one isn't so important, don't
bother with compatibility.