Update to glTF-Transform 4

[javagl]

Addresses #127

Opened as a draft PR for now. Details will be added here soon.

[donmccurdy]

Hi @javagl! Just curious if there's anything I can do to help get this PR ready? I've been doing some testing with EXT_mesh_features in glTF Transform v4 today, and that much seems to be working as expected.

[javagl]

Thanks @donmccurdy (I somehow expected that this PR might catch you attention sooner or later 😀 ).

I'm about to shift a bit of priority towards the 3D Tiles Tools right now. I'll first try to get #189 done (and tested in the validator). But the glTF-Transform upgrade is next on the list.

This has been pending for a while. So I may need to refresh my memory of the current state here, and add the corresponding details in the first post. (Good that I used that vague term "soon"...).

You may remember that donmccurdy/glTF-Transform#1367 (comment) was a key point here, and as mentioned in that comment: The corresponding change here, namely, replacing the replicated copyToDocument/mergeDocuments functions here with the original ones from glTF-Transform, seemed to work smoothly.

So I'll try to revive this PR as soon as the other one is done (and update from 4.1.4 to 4.3.0 while I'm at it...).

I'll tag you soon here (hopefully only with an "It's done (optionally review)" 🙂 )