performance improve

Do these things

* use `auto&` instead of `auto` if possible
* correct some coding style
* initialize an object in constructor to remove unneeded assign operation
* remove unneeded temple object
In fact, it is better to keep a 3d position.
But as you know, cocos2d-x originally is a 2d game engine, so there is _position(which is a Vec2), and _position is used in many places which means we can not remove it.

If we add a _position3D, then there will be redundant information in engine.