将ArcBall应用到相机变换
尝试通过相机位置和观察目标来控制相机

Signed-off-by: yhexie <yhlx125@163.com>
Merge branch 'master' of https://github.com/yhexie/RGeos.SlimScene

Conflicts:
	RGeos.SlimScene.suo
	RGeos.SlimScene/Controls/UcSceneControl.cs