Sphere occluders (portals and general use)

Add framework for supporting geometrical occluders within rooms, and add support for sphere occluders.
Includes gizmos for editing.

They also work outside the portal system.

core/engine.h

@@ -61,6 +61,7 @@ private:
 	uint64_t _physics_frames;
 	float _physics_interpolation_fraction;
 	bool _portals_active;
+	bool _occlusion_culling_active;
 
 	uint64_t _idle_frames;
 	bool _in_physics;