function distanceOfPointFromPlane(positionOnPlane, planeNormal, pointToTest) {
  // the d value in the plane equation a*x + b*y + c*z=d
  var d = planeNormal.dot(positionOnPlane);

  // distance of point from plane
  return (d - planeNormal.dot(pointToTest)) / planeNormal.length();
}
Find the distance from a plane defined by a point on the plane and the normal of the plane to any point.
@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
@returns {number}