xwm: Just ignore the synthetic unmap_notify
xwm: Just ignore the synthetic unmap_notify

As per ICCCM 4.1.4 we're supposed to withdraw the window when we see
the real unmap or the synthetic unmap, whichever comes first.  The
synthetic unmap may come after the window has been destroyed, so let's
just only handle the real unmap.

Tarnyko/weston-xdg_surface_present,Gnurou/weston,jonnylamb/weston,kwm81/weston,giucam/weston,giucam/weston,Gnurou/weston,Fantu/compositor-spice,Fantu/compositor-spice,xorgy/weston,eyolfson/weston,jonnylamb/weston,mchalupa/weston,Tarnyko/weston-xdg_surface_present,xorgy/weston,udoprog/weston,krezovic/weston,krezovic/weston,sir-murray/weston,udoprog/weston,kwm81/weston,eyolfson/weston,Fantu/compositor-spice,sir-murray/weston,mchalupa/weston