This implements a suggestion made in
https://github.com/floe/backscrub/issues/133#issuecomment-1085598710 to
address what appears to be a problem in the v4l2loopback driver.
And better aligns OBS' usage to the v4l2 kernel interface.
fixes#4808
Co-authored-by: Norihiro Kamae <norihiro@nagater.net>