Fix MJPEG white balance information when VA-API is in use.

Fix MJPEG white balance information when VA-API is in use.

The JPEG headers were cached (per resolution) and never invalidated,
causing wrong/outdated information to be sent.

I can't really remember why I wanted to cache these, but I suppose
I had a reason, so I'm a bit reluctant to just kill the cache.
Hopefully, the white balance won't change often enough, and these
objects are so small, that we won't need invalidation; we can just
let it grow until program exit.