Fix extended_material_bindless example wgpu validation error on MacOS/Metal

[NicoZweifel]

Objective

• Fixes extended_material_bindless example wgpu validation error on MacOS/Metal #21719

Solution

• Implementing bindless_supported of the AsBindGroup trait for ExtendedMaterial:

    fn bindless_supported(render_device: &RenderDevice) -> bool {
        B::bindless_supported(render_device) && E::bindless_supported(render_device)
    }

Before it just returned the default (true), leading to the mismatch.

I made small, mostly stylistic, improvements to fn bind_group_layout_entries and a few other locations like a comment in extended_material during debugging but I could move that out into a separate PR as well. Since the changes are rather limited and related to the issue I decided to leave it in for now.

Testing

• Tested on Windows/NixOS and MacOS, tested multiple examples but most testing happened on the extended_material_bindless and extended_material examples.

[mgi388]

I know there are already approvals, but I tested on my Macbook Pro and it fixes the bug for me so leaving another approval.

INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "macOS 15.7.2 Sequoia", kernel: "24.6.0", cpu: "Apple M1 Max", core_count: "10", memory: "64.0 GiB" }
INFO bevy_render::renderer: AdapterInfo { name: "Apple M1 Max", vendor: 0, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Metal }
INFO bevy_render::batching::gpu_preprocessing: GPU preprocessing is fully supported on this device.

I ran these examples (added the last 2 just because I tested them but I don't think they'd be affected anyway):

cargo run --example extended_material_bindless
cargo run --example extended_material
cargo run --example texture_binding_array
cargo run --example storage_buffer
cargo run --example array_texture