Fix crash when GfxManagedMetalLayerDelegate's backing Objective-C class has already been registered

[yamadapc]

Fixes a crash when GfxManagedMetalLayerDelegate objective-c class is already registered.

I'm sure there's a better solution, let me describe the issue:

• I'm writing an application which hosts audio plugins & uses iced, internally it's using gfx
• I've an audio plugin I've written which also uses iced & uses gfx internally too
• When the plugin is loaded into the host, this class name has already been registered and thus registration fails
• Unwrap on registration bits causes a crash

I really don't know what this code is trying to do, but as far as registering the objective-c class decl, I suppose there're a couple of options:

• Declare an unique name every time
• Don't declare the class if it's present
  • Issue: plugins may not have the same version of gfx-backend-metal, so the class decl needs to be binary compatible
• Declare a versioned class name

Let me know what you think & I'm happy to change the PR to go with another option. I suppose an unique class name is a good way to avoid problems, but that can be a bit messy if this class decl will be read somewhere else by name.

PR checklist:

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

[kvark]

thank you! 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