WayVR: External process support, various tweaks and bugfixes

- Support for foreign wayland clients, WayVR process is now separated into
  managed and external one
- Add `run_compositor_at_start` global param
- Add `primary` display param
- Export WAYLAND_DISPLAY number into XDG_RUNTIME_DIR directory
- Bugfix: Redraw event is not triggered after despawning a process
- Sanitization in WayVRConfig::post_load()

src/backend/openvr/mod.rs

@@ -328,9 +328,7 @@ pub fn openvr_run(running: Arc<AtomicBool>, show_by_default: bool) -> Result<(),
         };
 
         #[cfg(feature = "wayvr")]
-        if let Some(wayvr) = &state.wayvr {
-            wayvr.borrow_mut().tick_events()?;
-        }
+        crate::overlays::wayvr::tick_events::<OpenVrOverlayData>(&mut state, &mut overlays)?;
 
         log::trace!("Rendering frame");