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.