Show ffmpeg progress

[Nannk]

Is your feature request related to a problem? Please describe.
As av1 encoding on a cpu takes quite a long time, in my opinion, a progress indicator would be quite helpful.

Describe the solution you'd like
ffmpeg has an -stats output and -progress output option. source
One of the solutions is to use ffprobe before ffmpeg, grab the frame counter from -progress output and calcutate the progress in percent. source.

There are also multiple wrappers available, most of them use js, python or php.
Bash implementation: https://github.com/zamanuhina/ffmpeg-progress

[bytedream]

Added in v3.2.0