Seems that maxrate needs somekinda exponential function from bitrate (comparing
ffmpeg's libvpx presets) and cbr isn't something you likely will get out of this.
(cherry picked from commit d3c422e310bfaed687554e2553357b89b3b2a82e)
Signed-off-by: Ilkka Ollakka <ileoo@iki.fi>