fix despawning of scenes (fixes framerate drop after traveling far)

src/world.rs

@@ -2,6 +2,7 @@ use crate::{actor, nature, settings};
 use bevy::prelude::*;
 use bevy::render::render_resource::{AsBindGroup, ShaderRef};
 use bevy::math::{DVec3, I64Vec3};
+use bevy::scene::{InstanceId, SceneInstance};
 use bevy_xpbd_3d::prelude::*;
 use bevy_xpbd_3d::plugins::sync::SyncConfig;
 use std::collections::HashMap;
@@ -78,6 +79,7 @@ struct AsteroidData {
 #[derive(Event)]
 pub struct DespawnEvent {
     entity: Entity,
+    sceneinstance: InstanceId,
     origin: I64Vec3,
 }
 
@@ -182,7 +184,7 @@ fn spawn_despawn_asteroids(
     mut ew_despawn: EventWriter<DespawnEvent>,
     mut db: ResMut<ActiveAsteroids>,
     asset_server: Res<AssetServer>,
-    mut q_asteroid: Query<(Entity, &mut Visibility), With<Asteroid>>,
+    mut q_asteroid: Query<(Entity, &SceneInstance), With<Asteroid>>,
     mut last_player_cell: Local<I64Vec3>,
 ) {
     if !timer.0.tick(time.delta()).just_finished()  || q_player.is_empty() {
@@ -218,11 +220,11 @@ fn spawn_despawn_asteroids(
             || origin.z < z_min || origin.z > z_max
         {
             let mut despawning_worked = false;
-            for (ent, mut vis) in &mut q_asteroid {
+            for (ent, sceneinstance) in &mut q_asteroid {
                 if ent == asteroid.entity {
-                    *vis = Visibility::Hidden;
                     ew_despawn.send(DespawnEvent {
                         entity: asteroid.entity,
+                        sceneinstance: **sceneinstance,
                         origin: origin.clone(),
                     });
                     despawning_worked = true;
@@ -336,9 +338,11 @@ fn handle_despawn(
     mut commands: Commands,
     mut er_despawn: EventReader<DespawnEvent>,
     mut db: ResMut<ActiveAsteroids>,
+    mut scene_spawner: ResMut<SceneSpawner>,
 ) {
     for despawn in er_despawn.read() {
         commands.entity(despawn.entity).despawn();
+        scene_spawner.despawn_instance(despawn.sceneinstance);
         db.0.remove(&despawn.origin);
     }
 }