shell: forbid multiple wl_shell_surface objects
shell: forbid multiple wl_shell_surface objects

Do not allow multiple wl_shell_surface objects to be created for a
wl_surface object.

Multiple shell_surface objects would confuse the compositor as they
contain separate instances of the shell-private data.

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

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