Write randomPointInCircle(radius, cx, cy, rng), which returns a point [x, y] chosen uniformly from the disc of the given radius around (cx, cy): a region of the disc has probability in proportion to its area.
rng returns numbers in [0, 1). Use only it for randomness- A point on the boundary counts as inside;
radius = 0 returns the center - Two good methods: draw a point in the bounding square and redraw until it is inside the circle, or draw an angle and a distance
radius * Math.sqrt(u) - A distance of
radius * u is wrong: it crowds points near the center