[feature request] Allow encoding through ffmpeg.

[lastrosade]

As per title: Allow us to use ffmpeg to encode directly.

With libaom-av1 and libvpx-vp9.

This would avoid some overhead and some piping related issues. It would also allow some testing.

[master-of-zen]

@lastrosade everything should be fine. Also overhead is not really a thing if you use ffsm2/lsmash

[lastrosade]

Allow us to specify the encoder we use in ffmpeg, this way we can not only have more choice but also remove dependencies.

[ahmadrr]

I tried using ffms2/lsmash but received error "index out of range", never got them working.