Use RenderStartup in MaterialPlugin (#19885)

# Objective

- The MaterialPlugin has some ugly code to initialize some data in the
render world
- #19887

## Solution

- Use the new RenderStartup schedule to use a system instead of using
the plugin `finish()`

## Testing

- Tested that the 3d_scene and shader_material example still work as
expected

Author: IceSentry

crates/bevy_pbr/src/material.rs

@@ -37,6 +37,7 @@ use bevy_render::erased_render_asset::{
 use bevy_render::mesh::mark_3d_meshes_as_changed_if_their_assets_changed;
 use bevy_render::render_asset::{prepare_assets, RenderAssets};
 use bevy_render::renderer::RenderQueue;
+use bevy_render::RenderStartup;
 use bevy_render::{
     batching::gpu_preprocessing::GpuPreprocessingSupport,
     extract_resource::ExtractResource,
@@ -371,44 +372,40 @@ where
         }
 
         if let Some(render_app) = app.get_sub_app_mut(RenderApp) {
-            render_app.add_systems(
-                ExtractSchedule,
-                (
-                    extract_mesh_materials::<M>.in_set(MaterialExtractionSystems),
-                    early_sweep_material_instances::<M>
-                        .after(MaterialExtractionSystems)
-                        .before(late_sweep_material_instances),
-                    extract_entities_needs_specialization::<M>.after(extract_cameras),
-                ),
-            );
-        }
-    }
-
-    fn finish(&self, app: &mut App) {
-        let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
-            return;
-        };
-
-        render_app.world_mut().resource_scope(
-            |world, mut bind_group_allocators: Mut<MaterialBindGroupAllocators>| {
-                let render_device = world.resource::<RenderDevice>();
-                bind_group_allocators.insert(
-                    TypeId::of::<M>(),
-                    MaterialBindGroupAllocator::new(
-                        render_device,
-                        M::label(),
-                        material_uses_bindless_resources::<M>(render_device)
-                            .then(|| M::bindless_descriptor())
-                            .flatten(),
-                        M::bind_group_layout(render_device),
-                        M::bindless_slot_count(),
+            render_app
+                .add_systems(RenderStartup, setup_render_app::<M>)
+                .add_systems(
+                    ExtractSchedule,
+                    (
+                        extract_mesh_materials::<M>.in_set(MaterialExtractionSystems),
+                        early_sweep_material_instances::<M>
+                            .after(MaterialExtractionSystems)
+                            .before(late_sweep_material_instances),
+                        extract_entities_needs_specialization::<M>.after(extract_cameras),
                     ),
                 );
-            },
-        );
+        }
     }
 }
 
+fn setup_render_app<M: Material>(
+    render_device: Res<RenderDevice>,
+    mut bind_group_allocators: ResMut<MaterialBindGroupAllocators>,
+) {
+    bind_group_allocators.insert(
+        TypeId::of::<M>(),
+        MaterialBindGroupAllocator::new(
+            &render_device,
+            M::label(),
+            material_uses_bindless_resources::<M>(&render_device)
+                .then(|| M::bindless_descriptor())
+                .flatten(),
+            M::bind_group_layout(&render_device),
+            M::bindless_slot_count(),
+        ),
+    );
+}
+
 /// A dummy [`AssetId`] that we use as a placeholder whenever a mesh doesn't
 /// have a material.
 ///