insprited by Unreal Spring Arm componet
local Position of MainCamera is (0,0,0)
CameraPivot offer the same function as SpringArm
But using Quaternion make the code become clear and simple
z value of MainCamera is length of SpringArm
notes:
these two component has problem when using on planet. (when player moves ,
camera will shak a little)the Problem of CameraPivot is fixed now!