Allow audio in the built-in video recording

[eira-fransham]

Motivation

Seismon has built-in functionality to record videos using the startvideo/stopvideo commands - the motivation behind this is to be able to directly record demos as videos, and to do things like record videos without a HUD. We use video-rs as our video backend right now, but it doesn't support audio.

Proposal

Directly use ffmpeg, or fork video-rs to support audio. We already have per-frame audio buffers available from fundsp::Snoop, so as long as we have some way to send audio to ffmpeg this shouldn't be too difficult.