* 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.
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