Add a function to clear bindgroups from passes and bundles, with stub backend implementations.

[bradwerth]

Since the spec allows null BindGroups to be passed to setBindGroup, we need a way to clear the BindGroup associated with an index. This patch implements that through a clear_bind_group function defined in various necessary places, and implements as much of it as possible, without getting into the wgpu-hal backend implementations, which are left as TODOs.

Connections
N/A

Testing
This code is not exercised, since all of our set_bind_group entry points require a BindGroup. The CTS contains tests that set null BindGroups.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[ErichDonGubler]

I have several questions before I'm willing to dive into a full review of the implementation:

1. Is this actually shippable without having hal implementations?
2. Is there a reason that we shouldn't adhere more to the design of the JS API, specifically WRT GPUDevice.setBindGroup expressing this use case as an optional bindGroup argument, rather than a separate method?

[bradwerth]

I have several questions before I'm willing to dive into a full review of the implementation:

1. Is this actually shippable without having `hal` implementations?

Probably not. Undoing the effects of a setBindGroup are subtle and require expertise per-backend; expertise I don't have.

2. Is there a reason that we shouldn't adhere more to the design of the JS API, specifically WRT `GPUDevice.setBindGroup` expressing this use case as an optional `bindGroup` argument, rather than a separate method?

Modifying the top-level entry point would be superior I think, but it would break all usage with the change, since every caller of setBindGroup would need to be modified. I also wasn't sure which of the many implementations of setBindGroup is the topmost, external-facing one. For those reasons, I offered the implementation up in this form.

[bradwerth]

I'll create a new PR that changes the external-facing signature of setBindGroup to take an Option<bindGroupId>.

[bradwerth]

The new PR is #6216.