Improper clipping in children of a retainer widget

Children of retainer widgets have improper clipping, causing any widgets set to "Clip to Bounds" to clip incorrectly. This may also affect hit testing in cases where multiple retainer widgets are nested, as the improper clip rect will be applied to any children of the outermost retainer.

1. Download the attached uasset (made in Release 5.5)
2. Open the widget in the editor and detach the UMG designer viewport into it's own window
3. Note that the widget clipping is applied with an offset (seemingly positioned based on the window coordinates instead of the virtual window coordinates)