compositor: assign output to drag surfaces
compositor: assign output to drag surfaces

Otherwise we endup with a surface that is mapped but with outtup nil.

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