Session Multi-GPU

Created by: ascent12

This is part of the effort to get multi-GPU support in the DRM backend. It removes the single-GPU assumptions from wlr_session code.

In order to do this, I needed to maintain a list of all of the devices we have open, which wlr_udev already maintained. Instead of maintaining two different lists of devices, or trying to create an extra interface between wlr_udev and wlr_session, I just merged them together. They are doing conceptually similar things and are always used together anyway.