WebGPURenderer: implement compileAsync()

[aardgoose]

Add compileAsync( scene, camera ) for WebGPU and WebGL backends.

With WebGL the KHR_parallel_shader_compile extension is used if available.

Added to webgpu_occlusion example to test/demonstrate use.

Note: targetScene support to be added

[RenaudRohlinger]

Looks good to me.
Just added a commit to forward the compile -> compileAsync via getter.

In the WebGLRenderer we throttle the check of completion with setTimeout(()=>{}, 10). In my opinion using a RAF is just fine so I would let it as it is in the WebGLBackend.

I will let you decide if we merge this one or not as it affects the WebGPU side @sunag.

[sunag]

I think that since compileAsync() will call updateBefore() in many situations we will have multiples render() calls being executed during a compileAsync() process. I think compileAsync() should force the compilation of the entire consecutive process of the render tree and avoid overloading the frame in which it was called that could happen in some cases.

[aardgoose]

@RenaudRohlinger I Overwrote you commit by mistake, To you want to re add.

[RenaudRohlinger]

@aardgoose if I remember correctly the only difference was that I replaced:

async compile( /*scene, camera, targetScene = null */ ) {

		console.warn( 'THREE.Renderer: .compile() is not implemented yet.' );

}

to:

get compile() { // @deprecated, r170

      console.warn( 'THREE.WebGPURenderer: The property .compile has been deprecated. Use .compileAsync instead.' );
      return this.compileAsync;

}

In order to follow the best practice of the repo.

Are we good regarding the comment of @sunag?

[sunag]

I restored the commits, I ended up saving because of a small correction I made. Let's merge :)

[LeviPesin]

I don't really like that we are introducing a new property just to get a constant... Maybe at least mark it private?

[LeviPesin]

I see that all other non-async methods just point to their async counterparts, why should we deprecate this one? /ping @sunag @aardgoose @RenaudRohlinger