profiles/base/package.use.mask: mask mpv[libav]

profiles/base/package.use.mask: mask mpv[libav]

Libav is basically unsupported by mpv, see upstream issues 3923, 3925.
Needs patch to even build, subs are broken and not supported anymore.

I cannot call such state of mpv[libav] stable with a clear conscious.
Upstream's position on libav support leaves little hope the situation
will improve in the future.

Sorry, but mpv[libav] users will have to migrate to ffmpeg, to another
player, send fixes upstream, or live with partly broken functionality.