/**
* Draw a filled square of side length 2r, centred on (x, y).
*
* @param x the x-coordinate of the centre of the square
* @param y the y-coordinate of the centre of the square
* @param r radius is half the length of any side of the square
* @throws RuntimeException if r is negative
*/
public static void filledSquare(double x, double y, double r) {
offscreen.fill(ShapeAssist.square(x, y, r));
draw();
}
