GLTFLoader: assign unique names to children of cloned refs in _getNod…

[Mirei3D]

…eRef() (fixes #30090)

Related issue: #30090

Description

When cloning a reference the children of the cloned reference have the same names as the children of the original reference. This causes duplicate names for example when dealing with meshes that have multiple primitives and that are instantiated multiple times.

This pull request attempts to change this by applying the same _instance_X suffix applied to the reference name to its children. Another possible solution might be to apply createUniqueName() on the names of cloned children. Let me know if another solution than the current is preferred and I'd be happy to change it.

[donmccurdy]

Changes to the mesh primitive names will be a breaking change for R3F's gltfjsx and Threlte's gltf. We can do that, but to avoid back-to-back breaking changes I think it probably needs to happen at the same time as:

• GLTFLoader: Conflicting mesh/primitive/geometry mappings #29768

Together I think the two changes will need some pretty careful testing to make sure GLTFLoader is producing a scene graph (structure, names, userDatas) that we are comfortable maintaining.

[Mirei3D]

Seems reasonable to me to batch these breaking changes together.
On our end we will just implement a work around for now, which updates duplicate names as a post-processing step, and migrate our name references once an official fix is in if needed.