CornerRadius and _drawRoundedRect for ScrollBar?

Was looking at adding a CornerRadius to the scrollbar. It would be nice for certain styled elements. Would this be something others would like?

Was thinking just duplicate the behavior for drawing a _drawRoundedRect from the Rectangle class and making sure the ScrollBar has all the supporting properties for it.

Do you want to make a PR for it ?

For sure, will do when I get some free time!