This simplifies the Wayland backend, fixes the build configuration for
it, ports it to xdg-shell stable, and reworks some false assumptions
from the original port.
This patch has been spun of the rather massive amount of work for adding input
devices into Monado. The interfaces feels somewhat stable now so could do
with another pair of eyes on them, before too much work is done on st/oxr.
For enabling Vulkan validation during runtime.
Add CMake option to disable Vulkan validation at build time.
By default the DEBUG extension is enabled but the validation layers
are not when the ENV variable is not set to true.