window.c: Check for cairo device failure the right way
window.c: Check for cairo device failure the right way

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