addCompositor(compositor, name = "default", activate = false) {
		// make sure there is no existing compositor with the same name
		if (typeof this.compositors.get(name) !== "undefined") {
			throw new Error("Invalid Compositor name");
		}

		// add the new compositor
		this.compositors.set(name, compositor);

		if (activate === true) {
			// set as active one
			this.setCompositor(name);
		}
	}
add a new compositor to this renderer
@param {Compositor} compositor - a compositor instance
@param {string} name - a name uniquely identifying this compositor
@param {boolean} [activate=false] - true if the given compositor should be set as the active one