fontselect: expose a fontprovider selection API

Allow the user of libass to select the font provider from ass_set_fonts. This
API change actually doesn't break client code which was passing `fc=1`; now
the same value will autodetect a usable font provider.

Also add an api to list available font providers as that is useful for client
code to show drop down menus with a font provider to choose from.
fontselect: expose a fontprovider listing API

This allows client code to query libass for the font providers it was compiled
with. It can be useful for clients so that they can show selection interfaces
to their users.
fontselect: use fallback fonts when querying font providers [2]

To be squashed on merge.