Add a bindless mode to AsBindGroup.

[pcwalton]

This adds the infrastructure necessary for Bevy to support bindless resources. Bindless resources significantly improve rendering performance by reducing wgpu and driver overhead considerably.

TODO: Write more here.

Copied and pasted from the new AsBindGroup documentation:

• bindless(COUNT)
  • This switch enables bindless resources, which changes the way Bevy
    supplies resources (uniforms, textures, and samplers) to the shader.
    When bindless resources are enabled, and the current platform supports
    them, instead of presenting a single instance of a resource to your
    shader Bevy will instead present a binding array of COUNT elements.
    In your shader, the index of the element of each binding array
    corresponding to the mesh currently being drawn can be retrieved with
    mesh[in.instance_index].material_bind_group_slot.
  • The purpose of bindless mode is to improve performance by reducing
    state changes. By grouping resources together into binding arrays, Bevy
    doesn't have to modify GPU state as often, decreasing API and driver
    overhead.
  • If bindless mode is enabled, the BINDLESS definition will be
    available. Because not all platforms support bindless resources, you
    should check for the presence of this definition via #ifdef and fall
    back to standard bindings if it isn't present.
  • See the shaders/shader_material_bindless example for an example of
    how to use bindless mode.

This should be ready except for documentation. I'm submitting it now as a draft since this shouldn't land before 0.15 is released anyway.

Here's a Tracy profile of submit_graph_commands of this patch and an additional patch (not submitted yet) that makes StandardMaterial use bindless. Red is those patches; yellow is main. The scene was Bistro Exterior with a hack that forces all textures to opaque. You can see a 1.47x mean speedup.

Migration Guide

• RenderAssets::prepare_asset now takes an AssetId parameter.
• Bin keys now have Bevy-specific material bind group indices instead of wgpu material bind group IDs, as part of the bindless change. Use the new MaterialBindGroupAllocator to map from bind group index to bind group ID.

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.