Create instance from worker

[JolifantoBambla]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
#1986
#1466
#2652

Description
On the WebGPU backend, Context::init() uses web_sys::window() to access navigator.gpu.
Since window is not available on web workers this function panics when called from a web worker (#1986).
This is solved by introducing a Global type the result of js_sys::global() is cast to.
This instance of Global is then used to check whether or not window() is defined to determine if the function was called in a worker context or not. If it's in a worker context, it will use WorkerGlobalScope instead of Window to access the gpu property.
I'm relatively new to rust and took the solution from here after stumbling upon this issue in wasm-bindgen, so it's maybe not the prettiest solution.
Let me know if you have a better solution in mind.

Testing
I added logging of adapter.get_info() to the hello example and am about to submit another PR over at https://github.com/gfx-rs/wgpu-rs.github.io that adds an example running the hello example in a webworker.
Note though, that this example doesn't work in Firefox since it doesn't support loading modules in workers. I tested it in Chromium on Linux.

[JolifantoBambla]

the example I used to test this is here: gfx-rs/wgpu-rs.github.io#24

[JolifantoBambla]

Hey, @cwfitzgerald, I fixed the formatting. Could you maybe try running the checks again? Thanks :)

[cwfitzgerald]

Re-run! If you see failures on nightly only, ignore them, it's an upstream issue.

[JolifantoBambla]

Thanks! Looks like it worked :)

[cwfitzgerald]

One comment, but otherwise looks good.

[DemiKal]

Do you have an example with Rust code on how you use a web worker with wgpu? This is an interesting approach, but doesn't #1932 already handle this with offscreencanvas? When using a web worker, don't you need an offscreencanvas to begin with rendering? Where do you get it?

Note that I am using 0.12 and maybe 0.13 does something different and I am overlooking something. Anyway, my approach with wgpu 0.12 would be to pass the offscreencanvas to a web worker and init wgpu with that.

//call this from javascript worker
//pass canvas from the main thread to web worker with canvas.transferControlToOffscreen()

#[wasm_bindgen]
pub async fn run(offscreen_canvas: web_sys::OffscreenCanvas) {
    let instance = wgpu::Instance::new(wgpu::Backends::PRIMARY);
    let surface = unsafe { instance.create_surface_from_offscreen_canvas(offscreen_canvas: ) };
    ...
}

[cwfitzgerald]

Nice!