* Expose EditorNavigationMeshGenerator as an engine singleton so users
  can generate navmesehes from `tool` scripts.

* Add support for generating navmeshes from static colliders. All
  collision shapes are supported except for Plane (since Plane is an
  infinite collider and navmeshes need to have finite geometry).

* When using static colliders as a geometry source, a layer mask can be
  specified to ignore certain colliders.

* Don't rely on global transform. It still should give the exact same
  results but allows for building navmeshes on nodes that are not in the
  tree (useful in `tool` scripts).

* Update navigation gizmos after every new bake.

This work has been kindly sponsored by IMVU.