Examples: Fix webgl_worker_offscreencanvas.

[Mugen87]

Related issue: #23255

Description

It seems the spec of Import Maps has not yet defined how they should work with Web Workers (see WICG/import-maps#2). Hence webgl_worker_offscreencanvas is currently broken.

This PR is a hotfix to make webgl_worker_offscreencanvas work again. However, the PR is essentially wrong since using import * as THREE from '../../../build/three.module.js'; does not allow the usage of example modules.

I'm afraid the missing import maps support in workers was not discussed in #23255.

/cc @marcofugaro

[Mugen87]

broken: https://threejs.org/examples/webgl_worker_offscreencanvas
fixed: https://raw.githack.com/Mugen87/three.js/dev62/examples/webgl_worker_offscreencanvas.html

[marcofugaro]

This PR looks good to me.

examples/jsm/offscreen/ is not supposed to be imported by users anyway.

[Mugen87]

Yeah, but the problem is the introduction of import maps breaks all usage of example modules in workers. For example it's not possible to use OrbitControls in a worker anymore like in the following demo:

HTML: https://github.com/mrdoob/three.js/blob/dev/manual/examples/offscreencanvas-w-orbitcontrols.html
Worker Script: https://github.com/mrdoob/three.js/blob/dev/manual/examples/shared-orbitcontrols.js

[marcofugaro]

Hmmm, might be worth looking into an import map polyfill for web workers, see guybedford/es-module-shims#206

[marcofugaro]

Looks like polyfill still needs some work to be able to funciton in web workers.

@Mugen87 do you think we could use skypack for that particular example in the meantime? Like this: https://jsfiddle.net/b9fcn3dL/

[Mugen87]

Yes, there is no other choice anyway. When the manual's editor is fixed (see #23363 (comment)), this approach can be applied to the worker demos.

However, I'm not sure what to tell devs who run into this issue with their app. Asking to stay with r136 is only acceptable if the polyfill works with workers soon (e.g. in one or two month). If not, we need a different solution.

I think we have to clarify if #23255 would have been merged if we would be aware of this restriction. If we say yes, we have to figure out something for the worker scenario.

[mrdoob]

Cherry-picked in master.

[mrdoob]

Thanks!