add missing overlay plugin file

Signed-off-by: Schmarni <marnistromer@gmail.com>

crates/bevy_openxr/src/openxr/features/overlay.rs

@@ -0,0 +1,95 @@
+use std::{mem, ptr};
+
+use bevy::prelude::*;
+use openxr::sys;
+
+use crate::{
+    next_chain::{OxrNextChainStructBase, OxrNextChainStructProvider},
+    openxr::exts::OxrEnabledExtensions,
+    session::{OxrSessionCreateNextChain, OxrSessionCreateNextProvider},
+};
+
+pub struct OxrOverlayPlugin;
+
+impl Plugin for OxrOverlayPlugin {
+    fn build(&self, app: &mut bevy::prelude::App) {
+        app.add_event::<OxrOverlaySessionEvent>();
+        app.init_resource::<OxrOverlaySettings>();
+        app.add_systems(First, add_overlay_info_to_chain);
+    }
+}
+
+#[derive(Resource)]
+pub struct OxrOverlaySettings {
+    pub session_layer_placement: u32,
+    pub flags: openxr::OverlaySessionCreateFlagsEXTX,
+}
+
+impl Default for OxrOverlaySettings {
+    fn default() -> Self {
+        OxrOverlaySettings {
+            session_layer_placement: 0,
+            flags: openxr::OverlaySessionCreateFlagsEXTX::EMPTY,
+        }
+    }
+}
+
+fn add_overlay_info_to_chain(
+    mut chain: NonSendMut<OxrSessionCreateNextChain>,
+    exts: Res<OxrEnabledExtensions>,
+    settings: Res<OxrOverlaySettings>,
+) {
+    if exts.other.contains(&"XR_EXTX_overlay\0".to_string()) {
+        chain.push(OxrSessionCreateInfoOverlay::new(
+            settings.flags,
+            settings.session_layer_placement,
+        ));
+    }
+}
+
+#[derive(Event, Clone, Copy, Debug)]
+pub enum OxrOverlaySessionEvent {
+    MainSessionVisibilityChanged {
+        visible: bool,
+        flags: openxr::OverlayMainSessionFlagsEXTX,
+    },
+}
+
+pub struct OxrSessionCreateInfoOverlay {
+    inner: sys::SessionCreateInfoOverlayEXTX,
+}
+impl OxrSessionCreateInfoOverlay {
+    pub const fn new(
+        flags: openxr::OverlaySessionCreateFlagsEXTX,
+        session_layers_placement: u32,
+    ) -> Self {
+        Self {
+            inner: sys::SessionCreateInfoOverlayEXTX {
+                ty: sys::SessionCreateInfoOverlayEXTX::TYPE,
+                next: ptr::null(),
+                create_flags: flags,
+                session_layers_placement,
+            },
+        }
+    }
+}
+impl Default for OxrSessionCreateInfoOverlay {
+    fn default() -> Self {
+        Self::new(openxr::OverlaySessionCreateFlagsEXTX::EMPTY, 0)
+    }
+}
+
+impl OxrNextChainStructProvider for OxrSessionCreateInfoOverlay {
+    fn header(&self) -> &OxrNextChainStructBase {
+        unsafe { mem::transmute(&self.inner) }
+    }
+
+    fn set_next(&mut self, next: &OxrNextChainStructBase) {
+        self.inner.next = next as *const _ as *const _;
+    }
+    fn clear_next(&mut self) {
+        self.inner.next = ptr::null();
+    }
+}
+
+impl OxrSessionCreateNextProvider for OxrSessionCreateInfoOverlay {}