xrWaitFrame() doesn't block at all, will only predict the time for the next frame.
Once the compositor is multithreaded, this wait should be moved to the compositor thread
and xrWaitFrame should be the function to block until the compositor finished rendering the frame.
```txt
Conditional jump or move depends on uninitialised value(s)
at 0x6A5A7A0: vk_select_physical_device (comp_vk.c:847)
by 0x6A5AAF9: vk_create_device (comp_vk.c:916)
by 0x6A5CD82: compositor_check_vulkan_caps (comp_compositor.c:401)
by 0x6A5D659: comp_compositor_create (comp_compositor.c:656)
by 0x6A5F99B: xrt_gfx_vk_provider_create (comp_glue_vk.c:57)
by 0x6A78197: oxr_session_populate_vk (oxr_session_vk.c:32)
by 0x6A77E59: oxr_session_create_impl (oxr_session.c:536)
by 0x6A77F53: oxr_session_create (oxr_session.c:567)
by 0x6A6EA74: oxr_xrCreateSession (oxr_api_session.c:44)
by 0x2632A8: xrCreateSession (in hello_xr)
by 0x239F0D: (anonymous namespace)::OpenXrProgram::InitializeSession() (in hello_xr)
by 0x22ED25: main (in hello_xr)
Uninitialised value was created by a stack allocation
at 0x6A5CB05: compositor_check_vulkan_caps (comp_compositor.c:349)
```
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.
