Sndio support in MPV

Hi!

I don't keep up with MPV [1] development, so I was quite surprised that
the latest release dropped support for sndio [2]. I was looking into
whether it would be possible to simply revert the commit, but it turns
out they made quite a few changes after it, so additional fixes are
needed. I already "fixed" it enough to at least build successfully, but
it doesn't yet work, and I will admit that I don't really understand all
that's going on in that codebase.

- [1] https://github.com/mpv-player/mpv
- [2] https://github.com/mpv-player/mpv/commit/71d218eae4b4d93ada34ff74906f71ad359c84bc

So, in order to avoid repeated work, I was wondering if anyone already
tried to go this route? I couldn't find any open PRs in their
repository.

My hope is that they would consider adding the backend back once someone
has done the work of porting it over to their new architecture. If
anyone is pessimisitic about that, let me know too, I guess :P

Thanks,
Érico