gphoto2 support

[jcelerier]

The goal is to be able to use libgphoto2 to get real-time livestream of a plugged-in full-body USB camera (e.g. not a webcam that exposes /dev/videoX but something like a fujifilm, canon or nikon that comes with a proprietary video frame protocol or a non-standard UVC implementation).

One can look into what OBS studio is doing as we want pretty much the same thing.

Check for cameras supporting "Liveview" here: http://www.gphoto.org/proj/libgphoto2/support.php

The implementation would likely be very similar to the NDI, Spout, Spyhon etc.