shell: Center fullscreen surface whenever new buffer is attached
shell: Center fullscreen surface whenever new buffer is attached

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