addLevel method
object - The page:Object3D to display at this level.
distance - The distance at which to display this level of
detail. Default 0.0.
Adds a mesh that will display at a certain distance and greater. Typically the further away the distance, the lower the detail on the mesh.
Implementation
LOD addLevel([Object3D? object, double distance = 0]){
		this.distance = distance.abs();
		final levels = this.levels;
		int l;
		for ( l = 0; l < levels.length; l ++ ) {
			if ( distance < levels[l].distance ) {
				break;
			}
		}
		levels.insert(l, LOD(distance: distance, object: object));
		add(object);
		return this;
	}