Method roundedRect

src/main/java/field/graphics/FLine.java:495–498

draws a rounded rectangle with corner `x, y` and width `w` and heght `h` and corner radius `r`

(double x, double y, double w, double h, double r)

	/**
	 * draws a circle at `x, y` with radius `r`.
	 */
	public FLine circle(double x, double y, double r) {
		double k = 0.5522847498f * r;
		this.moveTo(x, y - r);
		this.cubicTo(x + k, y - r, x + r, y - k, x + r, y);
		this.cubicTo(x + r, y + k, x + k, y + r, x, y + r);
		this.cubicTo(x - k, y + r, x - r, y + k, x - r, y);