The bindless PR (#16368) broke some examples:
* `specialized_mesh_pipeline` and `custom_shader_instancing` failed
because they expect to be able to render a mesh with no material, by
overriding enough of the render pipeline to be able to do so. This PR
fixes the issue by restoring the old behavior in which we extract meshes
even if they have no material.
* `texture_binding_array` broke because it doesn't implement
`AsBindGroup::unprepared_bind_group`. This was tricky to fix because
there's a very good reason why `texture_binding_array` doesn't implement
that method: there's no sensible way to do so with `wgpu`'s current
bindless API, due to its multiple levels of borrowed references. To fix
the example, I split `MaterialBindGroup` into
`MaterialBindlessBindGroup` and `MaterialNonBindlessBindGroup`, and
allow direct custom implementations of `AsBindGroup::as_bind_group` for
the latter type of bind groups. To opt in to the new behavior, return
the `AsBindGroupError::CreateBindGroupDirectly` error from your
`AsBindGroup::unprepared_bind_group` implementation, and Bevy will call
your custom `AsBindGroup::as_bind_group` method as before.
## Migration Guide
* Bevy will now unconditionally call
`AsBindGroup::unprepared_bind_group` for your materials, so you must no
longer panic in that function. Instead, return the new
`AsBindGroupError::CreateBindGroupDirectly` error, and Bevy will fall
back to calling `AsBindGroup::as_bind_group` as before.
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