Rounded Rectangle Impl

How can we implement Rounded Rectangle?

It may not be good to directly create a struct `RoundedRectangle` similar to `Square` because it imports 90 degree Circle (maybe bezier curve is better).

Maybe we can define `RoundedRectangle` as `VItem::from_vpoints`.

ref: [https://blog.csdn.net/qq_42278240/article/details/136235412](https://blog.csdn.net/qq_42278240/article/details/136235412)