Add many_meshlet_materials stress test example (#23792)

# Objective

- Introduce a dedicated example to measure performance overhead when
dealing with a high number of meshlet instances and unique materials.

## Solution

- Added a new example `many_meshlet_materials` that spawns a
configurable grid of meshlet bunnies.

## Testing

- `SystemInfo { os: "Linux (CachyOS Linux rolling)", kernel:
"6.19.10-1-cachyos", cpu: "AMD Ryzen 7 5800H with Radeon Graphics",
core_count: "8", memory: "15.0 GiB" }`
- `AdapterInfo { name: "AMD Radeon Graphics (RADV RENOIR)", vendor:
4098, device: 5688, device_type: IntegratedGpu, device_pci_bus_id:
"0000:03:00.0", driver: "radv", driver_info: "Mesa 26.0.3-arch2.2",
backend: Vulkan, subgroup_min_size: 64, subgroup_max_size: 64,
transient_saves_memory: false }`
- `cargo run --features=meshlet,https --release --example
many_meshlet_materials`

---

## Showcase

Example works perfectly with 5x5 grid.

<img width="1024" height="599" alt="image"
src="https://github.com/user-attachments/assets/948b42b7-e88e-4a91-9fb7-f1c1b4d27e32"
/>

The issue begins when grid count increases to 10x10. Meshes start
flickering.

<img width="1024" height="598" alt="image"
src="https://github.com/user-attachments/assets/055df47f-39dc-41bc-be05-1978ec352e3a"
/>

With 50x50 meshes, the whole screen flickers.

<img width="1024" height="595" alt="image"
src="https://github.com/user-attachments/assets/7d685a2b-7389-43fe-b04e-a5987136f459"
/>

When running with unique materials, performance significantly drops on
my hardware.

<img width="2224" height="1298" alt="image"
src="https://github.com/user-attachments/assets/8d5b4677-7851-4234-9467-992f8db62a9d"
/>

---------

Co-authored-by: Carter Anderson <mcanders1@gmail.com>
Co-authored-by: François Mockers <francois.mockers@vleue.com>