function nearestPointInPlane(positionOnPlane, planeNormal, pointToTest, resultPoint) {
  var t = distanceOfPointFromPlane(positionOnPlane, planeNormal, pointToTest);
  // closest point on the plane
  resultPoint.copy(planeNormal);
  resultPoint.multiplyScalar(t);
  resultPoint.add(pointToTest);
  return resultPoint;
}
Find the point on a plane that lies closest to
@param {THREE.Vector3} positionOnPlane any point on the plane.
@param {THREE.Vector3} planeNormal the normal of the plane
@param {THREE.Vector3} pointToTest point to test
@param {THREE.Vector3} resultPoint where to store the result.
@returns {THREE.Vector3}