This commit contributes to filling the unpainted region unoccupied by
the menu bar and the client area in a window, including the MenuItem
hover overlay remains in the area after hovering over them as sometimes
MenuItems have their height more than the height of the menu bar,
leading to painting in the unmanaged region - hence, contributing to an
artifact line.

contributes to
https://github.com/eclipse-platform/eclipse.platform.swt/issues/62 and
https://github.com/eclipse-platform/eclipse.platform.swt/issues/127