This adds an "auto" choice to the concurrent-frames suboption, and makes
it the default.
I'm not so sure about making this the default, though. It could lead to
excessive buffering with large CPU counts. But we'll see.
This is basically a hack; but apparently a needed one, since many
vapoursynth filters insist on having a FPS set.
We need to apply the FPS override before creating the filters. Also
change some terminal output related to the FPS value.
This is an ancient filter, and we assume it's not useful anymore.
If you really want this, it's still available in libavfilter (e.g. via
--vf=lavfi=[pp...]). The disadvantage is that mpv doesn't pass through
QP information to libavfilter. (This was probably the reason vf_pp still
was part of mpv - it was slightly easier to pass QP internally.)
