update(dt) {
		let isDirty = false;

		// update the camera/viewport
		// iterate through all cameras
		this.cameras.forEach((camera) => {
			if (camera.update(dt) === true) {
				isDirty = true;
			}
		});

		// update all lights
		this.lights.forEach((light) => {
			if (light.update(dt) === true) {
				isDirty = true;
			}
		});

		return isDirty;
	}
update function
@name update
@memberof Stage
@ignore
@param {number} dt - time since the last update in milliseconds.
@returns {boolean}