xdg-shell: xdg_toplevel.error.invalid_parent isn't sent, allowing stack overflow in desktop-shell

Can be tested with https://gitlab.freedesktop.org/vyivel/randfall/-/blob/main/cases/xdg_toplevel_parent_cycle.c. If modified to attach larger buffers and keep running indefinitely, focusing any of the two surfaces causes activate() in desktop-shell/shell.c to recursively call itself.