Merge: gamnit & model_viewer: use full model dimensions and show many custom models

This PR improves gamnit with new attributes on `Model`: `center, dimensions, min & max`, and a rewrite of the same services on `Mesh`. These are used to better center the model in model_viewer.

model_viewer has also been improved with support for loading many custom models, and fixes to the VR variant.

Pull-Request: #2204
Reviewed-by: Jean Privat <jean@pryen.org>