getChildByGUID(guid) {
		const obj = this.getChildByProp("GUID", guid);
		return obj.length > 0 ? obj[0] : null;
	}
return the child corresponding to the specified GUID<br>
note : avoid calling this function every frame since
it parses the whole object list each time
@param {string|RegExp|number|boolean} guid - child GUID
@returns {Renderable} corresponding child or null