Support changing corner radius per individual corner

Right now, it's only possible to change the corner radius on all corners of a shape at once.

It would be very useful if it were possible to do so per corner.

I would also love to do so. Currently I'm trying to build a fancy LCARS (Star Trek GUI) interface and the shapes which are used there you be just two shapes if we would have the option for each corner radius. Currently I have to build a mask shape for the final size, a shape with the "shoulder" and another shape for the second cutout which I can't do because just one is possible in OverlayGroup.