Cleanup render schedule (#7589)

# Objective

- Fixes https://github.com/bevyengine/bevy/issues/7531

## Solution

- Add systems to prepare set
- Also remove a unnecessary apply_systems_buffers from ExtractCommands set.

crates/bevy_pbr/src/lib.rs

@@ -284,6 +284,7 @@ impl Plugin for PbrPlugin {
             // because prepare_lights creates new views for shadow mapping
             .add_system(
                 apply_system_buffers
+                    .in_set(RenderSet::Prepare)
                     .after(RenderLightSystems::PrepareLights)
                     .before(ViewSet::PrepareUniforms),
             )

crates/bevy_pbr/src/material.rs

@@ -196,7 +196,11 @@ where
                 .init_resource::<RenderMaterials<M>>()
                 .init_resource::<SpecializedMeshPipelines<MaterialPipeline<M>>>()
                 .add_system_to_schedule(ExtractSchedule, extract_materials::<M>)
-                .add_system(prepare_materials::<M>.after(PrepareAssetLabel::PreAssetPrepare))
+                .add_system(
+                    prepare_materials::<M>
+                        .in_set(RenderSet::Prepare)
+                        .after(PrepareAssetLabel::PreAssetPrepare),
+                )
                 .add_system(queue_material_meshes::<M>.in_set(RenderSet::Queue));
         }
 

crates/bevy_render/src/lib.rs

@@ -112,7 +112,6 @@ impl RenderSet {
         let mut schedule = Schedule::new();
 
         // Create "stage-like" structure using buffer flushes + ordering
-        schedule.add_system(apply_system_buffers.in_set(ExtractCommands));
         schedule.add_system(apply_system_buffers.in_set(PrepareFlush));
         schedule.add_system(apply_system_buffers.in_set(QueueFlush));
         schedule.add_system(apply_system_buffers.in_set(PhaseSortFlush));

crates/bevy_sprite/src/mesh2d/material.rs

@@ -161,8 +161,8 @@ where
                 .add_system_to_schedule(ExtractSchedule, extract_materials_2d::<M>)
                 .add_system(
                     prepare_materials_2d::<M>
-                        .after(PrepareAssetLabel::PreAssetPrepare)
+                        .in_set(RenderSet::Prepare)
-                        .in_set(RenderSet::Prepare),
+                        .after(PrepareAssetLabel::PreAssetPrepare),
                 )
                 .add_system(queue_material2d_meshes::<M>.in_set(RenderSet::Queue));
         }