From the xdg-shell specification:
If the parent is unmapped then its children are managed as
though the parent of the now-unmapped parent has become the
parent of this surface. If no parent exists for the now-unmapped
parent then the children are managed as though they have no
parent surface.
Since the fullscreen request may be made before the toplevel's surface
is mapped, the requested fullscreen output needs to be stored so it
can be retrieved on map (along with the existing fullscreen property).
This commit makes the required changes for wlr_xdg_toplevel.
ie. don't destroy surface->toplevel on xdg_toplevel destroy. Instead do this on
xdg_surface destroy.
This allows compositors to add toplevel listeners when the surface appears and
remove them when the surface is destroyed.