Fixes that NavigationPolygon did not update in all cases because the internal NavigationMesh was not always unref and freed which blocked creating a new mesh later if nothing else freed it.
Fixes navigation polygon data corruption caused by thread-use that changed vertices or polygons while the navigation polygon was processed, e.g. by server region sync, navmesh baking or user thread updates.
Rémi Verschelde
Patrick Sean Klein
smix8
Aaron Franke