Xwayland: add DRM leasing support

This is based on the unstable DRM leasing protocol currently under
review on wayland-devel. One unfortunate problem with this approach is
that we have to open the DRM node ourselves and enumerate the modes of
the connectors available for lease. We'll be able to refactor this
somewhat once the kernel supports zero-object DRM leases, which I intend
to implement in the near future.

This takes the approach suggested by Pekka in which no attempt is made
to correlate leasable connectors with a wl_output/xdg_output.
Accordingly, this also assumes that all leasable connectors are
non-desktop outputs.

Successfully tested with xrgears -w direct and SteamVR on an HTC Vive.
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