windows/gl: Fix double window class registration

[Azorlogh]

Description

On windows + gl backend:

• When a process loads a dynamic library containing wgpu, the gles backend will create a window class.
• It then leaks the window class (never unregisters it)
• If the library is unloaded and loaded again by that same process, wgpu will try to initialize the window class again, with the same name, and fail with: wgpu_core::instance: Instance::new: failed to create Gl backend: InstanceError { message: "unable to register window class", source: Some(Error { code: HRESULT(0x80070582), message: "Class already exists." }) }

This adds an additional check to skip initialization if the window class already exists.

Testing

I ran my wgpu-based VST plugin against this PR and it now works properly.
Without this change, it only managed to initialize once, then subsequent initializations fail to get the adapter.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • [ ] --target wasm32-unknown-unknown
• Run cargo xtask test to run tests. <- I don't have a windows machine to run tests on... The aforementioned VST ran on wine
• If this contains user-facing changes, add a CHANGELOG.md entry.