Implement floating anchors

This is a follow-up to #618.

**Is your feature request related to a problem? Please describe.**

When handling globally distributed 3d-objects in a single overlay, one of the problems is that a single anchor-point isn't enough to maintain acceptable precision in all locations.

**Describe the solution you'd like**

To solve this, we could add a feature I would call "floating anchor". With this, users would provide the geo-reference of an object in a property `object.userData.geoPosition` ([see three.js documentation](https://threejs.org/docs/?q=object3d#api/en/core/Object3D.userData)) and the overlay would automatically calculate the anchored world-space position for those geo-coordinates.

The anchor-point could then be automatically updated from map-movement. With every update, the scene is traversed and world-space coordinates are recomputed from the specified geo-reference.