Draft: Implement a DRM leasing service
Exposes a D-Bus service on mutter to manage DRM leasing. This can be consumed by a portal backend to implement a VR portal.
This can be tested with a custom version of kmscube: https://gitlab.freedesktop.org/swick/kmscube/-/commits/wip/kms-leasing-mutter
The last commit marks a monitor with specific dimensions as non-desktop. If you don't have an output which is reported as non-desktop by KMS (i.e. a VR headset) you can mark it as one here by putting in the dimensions reported by drm_info.
The D-Bus API is very basic on purpose. I expect changes will be required for a good VR portal.