code: function bezierat (a, b, c, d, t) {
     return Math.pow(1-t, 3) * a +
          3 * t * Math.pow(1-t, 2) * b +
          3 * Math.pow(t, 2) * (1 - t) * c +
          Math.pow(t, 3) * d
  }
docs: @ignore
  
  Bezier cubic formula
  ((1 - t) + t)3 = 1