Previously the switch did not catch all cases issuing a quite large
warning. Also there was a const-ness warning for codecs on ffmpeg 5.0
that this addresses.
Previously we assumed mjpeg was always decoded to 422 but it seems some
cameras provide frames that decode to different pixel formats such as
420.
This change delays setting the obs frame pixel format until after we
have decoded the v4l2 frame.
fixes#5821