Add exposure compensation

[ckreibich]

Describe the feature you'd like to see added to the app

I love this app and have been using it for several years. My one gripe is that my setup is prone to brightness changes and using exposure lock in mid-call is awkward — I have to do something like hold my hand to the camera to give it neutral brightness, then lock it, and do that repeatedly over time.

It would be preferable for me to have a slider in the UI that allows me to crank up/down the brightness.

[akvadrako]

I would also like exposure compensation, but in the CLI.