[PATCH v3] unstable/drm-lease: DRM lease protocol support

[Drew DeVault]

I have submitted a Vulkan extension which takes advantage of this
protocol for Vulkan-based VR clients:

https://github.com/KhronosGroup/Vulkan-Docs/pull/1001

This can be combined with my mesa/radv implementation:

https://git.sr.ht/~sircmpwn/mesa

As well as my xrgears tree:

https://git.sr.ht/~sircmpwn/xrgears

To test this on your compositor with any OpenHMD-compatible headset:

http://www.openhmd.net/index.php/devices/

It'll probably work on any display you make availble for a DRM lease,
though I haven't tried it on anything other than a VR headset.

I'm going to hold off on sending everything upstream until we reach
consensus on the protocol and we can fast-track it to stable for
Vulkan's sake.