compositor: Unmap drag icon surface when its released
compositor: Unmap drag icon surface when its released

libwayland-server was changed to emit the new drag icon signal instead
of faking an attach event with a NULL buffer so this has to be done on
this side.

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