Imitate hand-drawn (imperfect) circles using cubic Bézier segments, based on a Bézier approximation to a perfect circle. TLDR: a good approximation to a quarter circle of unit radius is a cubic Bézier curve with control points P0 = (0,1), P1 = (c,1), P2 = (1,c), P3 = (1,0) where c = 0.551915024494. By varying the points in a sensible way, we can do a good impression of hand-drawn circles.