从上一帧到当前帧的独立于 timeScale 的时间间隔（以秒为单位）（只读）。
When called from inside MonoBehaviour's FixedUpdate, it returns the unscaled fixed framerate
delta time.
与 deltaTime 不同，该值不受 timeScale 的影响。
See Time and Frame Rate Management in the User Manual for more information about how this property relates to the other Time properties.