shell: Factor out common code to create fullscreen black surfaces
shell: Factor out common code to create fullscreen black surfaces

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