Support MJPEG encoding of planar Y'CbCr sources.

Support MJPEG encoding of planar Y'CbCr sources.

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.