Inherit layers from parent object when creating hand and controller models

[hybridherbst]

Description

Since controller models and hand meshes are created asynchronously at runtime (e.g. fetched after controller connection), there's no good way to set layers on them (one would have to poll until they exist, or there would need to be an event for full creation being complete).

This PR makes them inherit layers from their parent objects on creation.

This contribution is funded by 🌵 needle.

[mrdoob]

Do you mind giving some more context of why is this needed?
Why do controllers need to be aware of layers?

[hybridherbst]

Sure - since layers control both visibility and raycasting, not being able to set the layers of loaded models is problematic when that is actually used. We've run into this issue with an app where layer 0 by default is raycastable (raycasting against the scene), and then the controller's rays are hitting themselves because we can't move them to another layer (since they're loaded async with no way to react to the loading being done).

What we had tried before this PR was to set the layer on the loader, but that didn't work because the loaded controller didn't inherit it - thus this PR. I guess the alternative would be to ensure that everything that dynamically loads models (e.g. the various XR factories) have proper callbacks to react to the loading being done.

(had to check the source code to see which of the loaders have async side effects and which directly load something)

[hybridherbst]

@mrdoob did you have a chance to look at this again? Anything else unclear? :)

[hybridherbst]

Rebased this on a recent dev (cleanly) in case there's still interest in merging this.
(we still need this / part of the reason why we're still using a three fork and not main)

[hybridherbst]

Rebased again; still interested in getting this merged. As layers are used for both visibility and raycasting it's quite important to be able to control them reliably.

cc @mrdoob

[hybridherbst]

Rebased again, still useful, would love to see this mini improvement in the core 🙂
cc @mrdoob

[hybridherbst]

Rebased again. With your recent interest in XR again maybe I can get your eyes on this @mrdoob? 🐰

[hybridherbst]

This issue here is similar to the current discussion with WebXR cameras - in some cases it's necessary to have correct layer setup and that's what this PR ensures on creation. I don't think this has unintended consequences.

[Mugen87]

Closing in favor of #27544.