Add many_materials stress test (bevyengine#17346)

# Objective

- This PR adds a new stress test called `many_materials` to benchmark
the rendering performance of many animated materials.
- Fixes bevyengine#11588 
- This PR continues the work started in the previous PR bevyengine#11592, which
was closed due to inactivity.

## Solution

- Created a new example (`examples/stress_tests/many_materials.rs`) that
renders a grid of cubes with animated materials.
- The size of the grid can be configured using the `-n` command-line
argument (or `--grid-size`). The default grid size is 10x10.
- The materials animate by cycling through colors in the HSL color
space.

## Testing

- I have tested these changes locally on my Linux machine.
- Reviewers can test the changes by running the example with different
grid sizes and observing the performance (FPS, frame time).
- I have not tested on other platforms (macOS, Windows, wasm), but I
expect it to work as the code uses standard Bevy features.

---

## Showcase

<details>
  <summary>Click to view showcase</summary>


![image](https://github.com/user-attachments/assets/b15209d4-f832-402b-a527-58e5048971d1)

</details>

Cargo.toml

@@ -2910,6 +2910,17 @@ description = "Displays many Text2d! Used for performance testing."
 category = "Stress Tests"
 wasm = true
 
+[[example]]
+name = "many_materials"
+path = "examples/stress_tests/many_materials.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.many_materials]
+name = "Many Animated Materials"
+description = "Benchmark to test rendering many animated materials"
+category = "Stress Tests"
+wasm = true
+
 [[example]]
 name = "transform_hierarchy"
 path = "examples/stress_tests/transform_hierarchy.rs"

examples/README.md

@@ -468,6 +468,7 @@ cargo run --release --example <example name>
 Example | Description
 --- | ---
 [Bevymark](../examples/stress_tests/bevymark.rs) | A heavy sprite rendering workload to benchmark your system with Bevy
+[Many Animated Materials](../examples/stress_tests/many_materials.rs) | Benchmark to test rendering many animated materials
 [Many Animated Sprites](../examples/stress_tests/many_animated_sprites.rs) | Displays many animated sprites in a grid arrangement with slight offsets to their animation timers. Used for performance testing.
 [Many Buttons](../examples/stress_tests/many_buttons.rs) | Test rendering of many UI elements
 [Many Cameras & Lights](../examples/stress_tests/many_cameras_lights.rs) | Test rendering of many cameras and lights

examples/stress_tests/many_materials.rs

@@ -0,0 +1,103 @@
+//! Benchmark to test rendering many animated materials
+use argh::FromArgs;
+use bevy::{
+    diagnostic::{FrameTimeDiagnosticsPlugin, LogDiagnosticsPlugin},
+    prelude::*,
+    window::{PresentMode, WindowPlugin, WindowResolution},
+};
+use std::f32::consts::PI;
+
+#[derive(FromArgs, Resource)]
+/// Command-line arguments for the `many_materials` stress test.
+struct Args {
+    /// the size of the grid of materials to render (n x n)
+    #[argh(option, short = 'n', default = "10")]
+    grid_size: usize,
+}
+
+fn main() {
+    // `from_env` panics on the web
+    #[cfg(not(target_arch = "wasm32"))]
+    let args: Args = argh::from_env();
+    #[cfg(target_arch = "wasm32")]
+    let args = Args::from_args(&[], &[]).unwrap();
+
+    App::new()
+        .add_plugins((
+            DefaultPlugins.set(WindowPlugin {
+                primary_window: Some(Window {
+                    resolution: WindowResolution::new(1920.0, 1080.0)
+                        .with_scale_factor_override(1.0),
+                    title: "many_materials".into(),
+                    present_mode: PresentMode::AutoNoVsync,
+                    ..default()
+                }),
+                ..default()
+            }),
+            FrameTimeDiagnosticsPlugin::default(),
+            LogDiagnosticsPlugin::default(),
+        ))
+        .insert_resource(args)
+        .add_systems(Startup, setup)
+        .add_systems(Update, animate_materials)
+        .run();
+}
+
+fn setup(
+    mut commands: Commands,
+    args: Res<Args>,
+    mesh_assets: ResMut<Assets<Mesh>>,
+    material_assets: ResMut<Assets<StandardMaterial>>,
+) {
+    let args = args.into_inner();
+    let material_assets = material_assets.into_inner();
+    let mesh_assets = mesh_assets.into_inner();
+    let n = args.grid_size;
+
+    // Camera
+    let w = n as f32;
+    commands.spawn((
+        Camera3d::default(),
+        Transform::from_xyz(w * 1.25, w + 1.0, w * 1.25)
+            .looking_at(Vec3::new(0.0, (w * -1.1) + 1.0, 0.0), Vec3::Y),
+    ));
+
+    // Light
+    commands.spawn((
+        Transform::from_rotation(Quat::from_euler(EulerRot::ZYX, 0.0, 1.0, -PI / 4.)),
+        DirectionalLight {
+            illuminance: 3000.0,
+            shadows_enabled: true,
+            ..default()
+        },
+    ));
+
+    // Cubes
+    let mesh_handle = mesh_assets.add(Cuboid::from_size(Vec3::ONE));
+    for x in 0..n {
+        for z in 0..n {
+            commands.spawn((
+                Mesh3d(mesh_handle.clone()),
+                MeshMaterial3d(material_assets.add(Color::WHITE)),
+                Transform::from_translation(Vec3::new(x as f32, 0.0, z as f32)),
+            ));
+        }
+    }
+}
+
+fn animate_materials(
+    material_handles: Query<&MeshMaterial3d<StandardMaterial>>,
+    time: Res<Time>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    for (i, material_handle) in material_handles.iter().enumerate() {
+        if let Some(material) = materials.get_mut(material_handle) {
+            let color = Color::hsl(
+                ((i as f32 * 2.345 + time.elapsed_secs()) * 100.0) % 360.0,
+                1.0,
+                0.5,
+            );
+            material.base_color = color;
+        }
+    }
+}