distanceTo(target) {
		const a = this.getBounds();
		let dx;
		let dy;

		if (target instanceof Renderable) {
			const b = target.getBounds();
			dx = a.centerX - b.centerX;
			dy = a.centerY - b.centerY;
		} else {
			// vector object
			dx = a.centerX - target.x;
			dy = a.centerY - target.y;
		}

		return Math.sqrt(dx * dx + dy * dy);
	}
return the distance to the specified target
@param {Renderable|Vector2d|Vector3d} target
@returns {number} distance