Clarify / unify instancing behavior of Mesh/Asset3D/Boxes/Ellipsoids

[Wumpf]

More concrete incarnation of

• Formalize component joins that are happening within visualizers #7026

As of writing Mesh & Asset3D will draw additional instances if there's more than one of LeafTransform3D components.

On the flip side Boxes3D and Ellipsoids3D will only draw as many boxes/ellipsoids ad there are HalfSize3D elements. Any additional LeafTransform3D components are ignored.

This is a bit inconsistent! Should we keep splatting out boxes/ellipsoids for each leaf transform or is Mesh/Asset's behavior not what we want after all?

Note that implementation of all of these isn't too challening, rather easy. We have to make a consistent & documented decision though. Ui is a bit more tricky to get right though (what does it mean to select the n'th index/instance?), we may want to create follow-up issues for that if needed.