compositor: Update only existing backgrounds in output_move
compositor: Update only existing backgrounds in output_move

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