libweston-desktop/xwayland: window type XWAYLAND cannot have a parent

Add an assert to ensure that a window of type XWAYLAND is never
attempted with a parent. Following the code though, the assert can be
made even stricter by allowing only TRANSIENT to have a parent.

This is essentially adding documentation.

v2: use stricter assert
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>

libweston-desktop/xwayland.c

@@ -74,6 +74,7 @@ weston_desktop_xwayland_surface_change_state(struct weston_desktop_xwayland_surf
 	bool to_add = (parent == NULL && state != XWAYLAND);
 
 	assert(state != NONE);
+	assert(!parent || state == TRANSIENT);
 
 	if (to_add && surface->added) {
 		surface->state = state;