options: make --ass-hinting a choice, instead of using magic numbers

options.c still doesn't use the constants defined by the libass headers,
but it's still better than exposing those to the user directly.
options: allow selecting the libass shaper

I'm using the word "languages" instead of "scripts" in the manpage, but
I think that's easier to understand with a smaller amount of
descriptions.
vaapi: fix compilation with Libav

The code using FFSWAP was moved from vo_vaapi.c to vaapi.c, which didn't
include libavutil/common.h anymore, just libavutil/avutil.h. The header
avutil.h doesn't include common.h recursively in Libav, so it broke
there.

Add FFSWAP as MPSWAP in mp_common.h (copy pasted from ffmpeg) to make
sure this doesn't happen again. (This kind of stuff happens all too
often, so screw libavutil.)