[mtl] Use Once for delegate initialization

[kvark]

Fixes the following errors in wgpu-rs test log:

objc[21181]: Class GfxManagedMetalLayerDelegate is implemented in both ?? (0x7f99dbe04520) and ?? (0x7f99dbd04520). One of the two will be used. Which one is undefined.

I think the reason is that class registration is not instantaneous, and not blocking. So with the current code (as of #3585) we could still try to register the same class twice.

PR checklist:

• [x] make succeeds (on *nix)
• [ ] make reftests succeeds
• [x] tested examples with the following backends: Metal

[kvark]

bors r+

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable