Code review comment for lp://staging/~vanvugt/mir/parenting-server-api

Hm. As mentioned on the client-side of this proposal I suspect it would make more sense for parent to _not_ be a surface attribute, but a create-time option; that should neatly resolve the problem that the int parameter of
59 + virtual void set_parent(std::weak_ptr<Surface> const&, int id) = 0;
is solving?