fillCircle(center, radius, color = Config.DEFAULT_FILL_COLOR) {
        this.ctx.beginPath();
        this.ctx.arc(center.x, center.y, Math.max(radius - 0.5, 0), 0, 2 * Math.PI);
        this.ctx.fillStyle = color;
        this.ctx.fill();
    }
Draws the inside of a circle.
@param {!Point} center The center of the circle.
@param {!number} radius The distance from the center of the circle to its side.
@param {!string=} color The fill color. Defaults to white.