feat: allow manually defining additional vulkan extensions

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

crates/bevy_openxr/examples/custom_vulkan_extensions.rs

@@ -0,0 +1,57 @@
+//! A simple 3D scene with light shining over a cube sitting on a plane.
+
+use bevy::{prelude::*, render::pipelined_rendering::PipelinedRenderingPlugin};
+use bevy_mod_openxr::{
+    add_xr_plugins,
+    graphics::{GraphicsBackend, OxrManualGraphicsConfig},
+};
+
+fn main() -> AppExit {
+    App::new()
+        .insert_resource(OxrManualGraphicsConfig {
+            fallback_backend: GraphicsBackend::Vulkan(()),
+            vk_instance_exts: vec![],
+            vk_device_exts: vec![ash::khr::external_memory::NAME],
+        })
+        .add_plugins(add_xr_plugins(
+            // Disabling Pipelined Rendering should reduce latency a little bit for button inputs
+            // and increase accuracy for hand tracking, controller positions and similar,
+            // the views are updated right before rendering so they are as accurate as possible
+            DefaultPlugins.build().disable::<PipelinedRenderingPlugin>(),
+        ))
+        .add_plugins(bevy_mod_xr::hand_debug_gizmos::HandGizmosPlugin)
+        .add_systems(Startup, setup)
+        .run()
+}
+
+/// set up a simple 3D scene
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    // circular base
+    commands.spawn((
+        Mesh3d(meshes.add(Circle::new(4.0))),
+        MeshMaterial3d(materials.add(Color::WHITE)),
+        Transform::from_rotation(Quat::from_rotation_x(-std::f32::consts::FRAC_PI_2)),
+    ));
+    // cube
+    commands.spawn((
+        Mesh3d(meshes.add(Cuboid::new(1.0, 1.0, 1.0))),
+        MeshMaterial3d(materials.add(Color::srgb_u8(124, 144, 255))),
+        Transform::from_xyz(0.0, 0.5, 0.0),
+    ));
+    // light
+    commands.spawn((
+        PointLight {
+            shadows_enabled: true,
+            ..default()
+        },
+        Transform::from_xyz(4.0, 8.0, 4.0),
+    ));
+    commands.spawn((
+        Camera3d::default(),
+        Transform::from_xyz(-2.5, 4.5, 9.0).looking_at(Vec3::ZERO, Vec3::Y),
+    ));
+}