staging/drm-lease: DRM lease protocol support

DRM leasing is a feature which allows the DRM master to "lease" a subset
of its DRM resources to another DRM master via drmModeCreateLease, which
returns a file descriptor for the new DRM master. We use this protocol
to negotiate the terms of the lease and transfer this file descriptor to
clients.

In less DRM-specific terms: this protocol allows Wayland compositors to
give over their GPU resources (like displays) to a Wayland client to
exclusively control.

The primary use-case for this is Virtual Reality headsets, which via the
non-desktop DRM property are generally not used as desktop displays by
Wayland compositors, and for latency reasons (among others) are most
useful to games et al if they have direct control over the DRM resources
associated with it. Basically, these are peripherals which are of no use
to the compositor and may be of use to a client, but since they are tied
up in DRM we need to use DRM leasing to get them into client's hands.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Signed-off-by: Drew DeVault <sir@cmpwn.com>
Signed-off-by: Xaver Hugl <xaver.hugl@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>