Cubic bezier approximation of a circular arc centered at the origin, from (radians) a1 to a2, where a2-a1 < pi/2. The arc's radius is r.

Returns an object with four points, where x1,y1 and x4,y4 are the arc's end points and x2,y2 and x3,y3 are the cubic bezier's control points.

This algorithm is based on the approach described in: A. Riškus, "Approximation of a Cubic Bezier Curve by Circular Arcs and Vice Versa," Information Technology and Control, 35(4), 2006 pp. 371-378.
function createSmallArc(r, a1, a2) {
    var a = (a2 - a1) / 2.0;
    var x4 = r * Math.cos(a);
    var y4 = r * Math.sin(a);
    var x1 = x4;
    var y1 = -y4;
    var q1 = x1 * x1 + y1 * y1;
    var q2 = q1 + x1 * x4 + y1 * y4;
    var k2 = 4 / 3 * (Math.sqrt(2 * q1 * q2) - q2) / (x1 * y4 - y1 * x4);
    var x2 = x1 - k2 * y1;
    var y2 = y1 + k2 * x1;
    var x3 = x2;
    var y3 = -y2;
    var ar = a + a1;
    var cos_ar = Math.cos(ar);
    var sin_ar = Math.sin(ar);
    return {
      x1: r * Math.cos(a1),
      y1: r * Math.sin(a1),
      x2: x2 * cos_ar - y2 * sin_ar,
      y2: x2 * sin_ar + y2 * cos_ar,
      x3: x3 * cos_ar - y3 * sin_ar,
      y3: x3 * sin_ar + y3 * cos_ar,
      x4: r * Math.cos(a2),
      y4: r * Math.sin(a2)
    };
  }