sub: workaround braindead libass API

libass won't use embedded fonts, unless ass_set_fonts() (called by
mp_ass_configure_fonts()) is called. However, we call this function when
the ASS_Renderer is initialized, which is long before the .ass file is
actually loaded. (I'm not sure why it tries to keep 1 ASS_Renderer, but
it always did this.)

Fix by calling mp_ass_configure_fonts() after loading them. This also
means this function will be called multiple times - hopefully this is
harmless (it will reinit fontconfig every time, though).

While we're at it, also initialize the ASS_Renderer lazily.

Fixes #1244.
umpv: update comment for the new FIFO location
osc: add seekbarstyle option
osc: add validation for string user options
man: document osc seekbarstyle option