Fix the material import on multimesh

[Simarilius-uk]

Seems totally broken, looks like the materials are getting imported but assigned to the wrong submesh. Used to work, so need to work out what broke it.
Also need to do something to deal with the .mesh.json thing that the new exporter does, as it does it on multimesh exports.
Need to have a think about how to handle multimesh exports, as dont need/want to import the seperate meshes as well as the merged bits if they've been done that way.

[DoctorPresto]

By multimesh import, do you mean just importing a mesh which was exported from Wolvenkit with multimesh or selecting multiple meshes at once on glb import? Does it still work as expected on the lts version?