For development to continue on systems (such as optimus laptops) that
have multiple GPUs, we need a way to reference all of the renderers
that have been created for devices in the system. A wlr_multi_gpu
struct holds renderers coming from two different sources:
1. the primary renderer (given to us by the compositor)
2. drm sub backends, each of which has a renderer created for
cross-GPU copies.
This change provides a way to access all of these from the same
place.
Sessions can now be retrieved from a backend in a more general manner.
Multi-backend gets back its `session` field that contains the session
if one was created, removing the interfacing from multi backend with the
drm backend directly. This adds the possibility to use sessions even
without the drm backend.
It additionally fixes the bug that 2 session objects got created when
WLR_BACKENDS were set to "libinput,drm".
To allow vt switching without drm backend (and drm fd) on logind, start
listening to PropertiesChanged signals from dbus and parse the session
"Active" property when no master fd was created (this does not change
current drm backend behaviour in any way).
