sub: make font provider user-selectable

libass had an API to configure this since 2013. mpv always used
ASS_FONTPROVIDER_AUTODETECT, because usually there's little reason to
use anything else. The intention of the now added option is to allow
users to disable use of system fonts.

I didn't consider it worth the trouble to add the coretext and
directwrite enum items from ASS_DefaultFontProvider. The "auto" choice
will have the same effect if they're available. Also, the part of the
code which defines the option does not necessarily have libass available
(it's still optional!), so defining all enum items as choices is icky. I
still added fontconfig, since that may be nice to emulate a nostalgic
2010 feeling of mpv freezing on fontconfig.

The option for OSD is even less useful. (But you get it for free, and
why pass up a chance to add yet another useless option?)

This is not quite what was requested in #6947, but as close as it gets.

sub/osd.c

@@ -64,6 +64,8 @@ static const m_option_t style_opts[] = {
     OPT_FLAG("italic", italic, 0),
     OPT_CHOICE("justify", justify, 0,
                ({"auto", 0}, {"left", 1}, {"center", 2}, {"right", 3})),
+    OPT_CHOICE("font-provider", font_provider, 0,
+               ({"auto", 0}, {"none", 1}, {"fontconfig", 2})),
     {0}
 };