code: function rotate(out, a, angle) {
    var c = Math.cos(angle),
        s = Math.sin(angle)
    var x = a[0],
        y = a[1]
  
    out[0] = x * c - y * s
    out[1] = x * s + y * c
  
    return out
  }
docs: Rotates a vec2 by an angle
  
  @param {vec2} out the receiving vector
  @param {vec2} a the vector to rotate
  @param {Number} angle the angle of rotation (in radians)
  @returns {vec2} out