code: function(point, out) {
          if (!out) {
              out = new Vector3();
          }
          var d = this.distanceToPoint(point);
          vec3.scaleAndAdd(out.array, point.array, this.normal.array, -d);
          out._dirty = true;
          return out;
      }
docs: Calculate the projection point on the plane
  @param  {clay.Vector3} point
  @param  {clay.Vector3} out
  @return {clay.Vector3}