Function rate_desc 

pub fn rate_desc(settings: &RustCaptureSettings, fixed: Option<&str>) -> String

The rate-control field of a video stream’s log line, as the session applies it: a backend with one rate control (fixed, from FrameEncoder::fixed_rate_control) runs it at the bitrate target whatever the mode, and every other session runs its mode, a CRF one with no bitrate cap.