notifyObservers = function () {
                var eventArgs = {
                    vrDisplay: _this._vrDisplay,
                    vrSupported: _this._vrSupported
                };
                _this.onVRDisplayChangedObservable.notifyObservers(eventArgs);
            }
force a specific size of the canvas
@param {number} width - the new canvas' width
@param {number} height - the new canvas' height