Fix a potential incompatibility with Compton: it might unmap the Composite Overlay Window.

If this should happen, we'll just map it again. Yes, this is still bad,
as that means that after unlocking the screen the output may be still
black - but at least one can workaround that by forcing Compton to
redirect again (e.g. by alt-tabbing around). Better than revealing
screen content despite lock.