Detect if configured encoder is not available in ffmpeg

[chrisbenincasa]

We should be able to detect whether the encoder users configure in the ffmpeg settings is actually available within ffmpeg

We can issue a command like ffmpeg -hide_banner -encoders. There are others available like this, for filters, decoders, etc.

[chrisbenincasa]

Similarly we could potentially expose available encoders via the api and some clever stdout parsing

[chrisbenincasa]

Superseded by #442