Avoid invariant checking spew in gnome-shell

Avoid invariant checking spew in gnome-shell

For normal toplevels, visible is tightly bound to mapped, but for
something like a toplevel that exists within a Clutter stage we
may want to make mapping dependenton external factors, so we shouldn't
actually checked that !mapped toplevels are !visible.

Pointed out by Owen Taylor,

https://bugzilla.gnome.org/show_bug.cgi?id=637834