GLTF: Make skeleton bone names unique per-skeleton instead of scene-wide

[aaronfranke]

Fixes #106073, fixes #93758, fixes #96248. This restores the behavior in Godot 4.2 and earlier.

This PR makes the unique name feature no longer apply cross-skeleton. Skeleton bones now need to be unique within the same skeleton and compared to scene nodes, but not compared to all skeletons in the scene node.

This only affects glTF files with multiple skeletons in it, which is a niche use case, so this is low-risk. Users of this feature expect to be able to use the same bone map for each skeleton for animation targeting.

Note: This PR makes it possible to generate multiple nodes with the same name in the case of BoneAttachment3D nodes, since they are given the same name as the bone they are attached to. I think this is fine, and the alternatives are worse.

Note: This PR does not affect export. A comment introduced in PR #53114 indicates the current export behavior of having all glTF nodes including bones be unique was intentional. We could change the export behavior too, though.

Note: This PR adds a duplicate() function to HashSet, because I needed to duplicate a HashSet.

[fire]

Did not review the code.

Approve of the feature.

[lyuma]

Seems good for glTF

[akien-mga]

Thanks!

[dmlary]

EDIT: There's a config option at the bottom of the import settings under glTF, called Naming Version. It needs to be set to "Godot 4.5 or later". I think it preserved the 4.3-4.4 naming when I opened an old project.

This works great, thank you so much!

I was looking for this fix on 4.5-beta1. It looks like it is present for GLTF import, but bones are still renamed when imported directly from .blender files. Does the blender import path leveraged GLTF behind the scenes? Is there any way we can get this fix in for .blender files too by the 4.5 release?