This enables MJPEG encoding for video sources, but in practice,
only SRT cameras are enabled. There's VA-API support for 4:2:0 only,
by means of converting to NV12; if you had 4:2:2 or 4:4:4, you will
get fallback to libjpeg, which should handle that gracefully.
(If I actually had an SRT source doing 4:2:2, I might have added
more support, but it seems pretty narrow.)
Futatabi seemingly has some problems handling these files, but that
should be fixable.