Fix export mesh lib break due to imported gltf has a redundant Node3D

[jsjtxietian]

Fixes #81850

The code is a little messy this way, but imho it's better to change the mesh lib export code than change the gltf import code. I would appreciate any feedback or alternative suggestions.

[fire]

@aaronfranke does this interfere with any of your changes?

[aaronfranke]

Long-term, I would like to adjust the mesh library import code so that it no longer needs a MeshInstance3D node. Instead, it should be able to just grab the mesh resources imported from the glTF file. That way it does not depend on the glTF file having any particular node structure, and can even work when the glTF file has no nodes.

Short-term, I don't have any objections to this ad-hoc fix.

[jsjtxietian]

Instead, it should be able to just grab the mesh resources imported from the glTF file.

Agreed, do we need a proposal for this ？Or add a todo in the comment.

[aaronfranke]

@jsjtxietian I would do this as part of godotengine/godot-proposals#7494, allowing MeshLibrary as one of the resource import options. However this rework is significant so it must wait for Godot 4.3 at the earliest. I think this PR is fine to merge for Godot 4.2 as an ad-hoc fix for now.

[YuriSizov]

Short-term, I don't have any objections to this ad-hoc fix.

I assume we'll need compatibility code for this?

[lyuma]

Is there a case where this change can cause compatibility breakage? My understanding is this PR allows one extra case where an object wouldn't have been considered legal for a mesh library.

But I do agree with the assessment that ideally all meshes in the glTF document will be used regardless of node structure.

[akien-mga]

Superseded by #87923.