compositor: fix sub-surface view stacking order

If you opened a window with sub-surfaces, and then raised another window
on top of that, the underlaying window's main surface was stacked
properly, but the sub-surfaces remained on top of the raised window.
IOW, the raised window was in between the other window and its
sub-surfaces.

This got broken in a7af70436b, "Split the
geometry information from weston_surface out into weston_view".

Fix the issues:

In view_list_add_subsurface_view(), the views need to be added to the
end of the list, not to the head. This alone fixes the above problem,
but causes the sub-surface views to be stacked irrespective of their
surface stacking order. The stacking order in this test case is fixed by
the changes to view_list_add(), but for sub-sub-surfaces a similar
change is needed in view_list_add_subsurface_view() too.

In view_list_add(), build the view list in the sub-surface stacking
order, instead of pulling the parent surface always on top. Also handle
the case, when the subsurface_list is completely empty: the parent
surface's view must still be added.

Reported-by: Julien Isorce <julien.isorce@collabora.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Jason Ekstrand <jason@jlekstrand.net>

src/compositor.c

@@ -1612,11 +1612,18 @@ view_list_add_subsurface_view(struct weston_compositor *compositor,
 	}
 
 	weston_view_update_transform(view);
-	wl_list_insert(compositor->view_list.next, &view->link);
 
-	wl_list_for_each(child, &sub->surface->subsurface_list, parent_link)
+	if (wl_list_empty(&sub->surface->subsurface_list)) {
-		if (child->surface != sub->surface)
+		wl_list_insert(compositor->view_list.prev, &view->link);
+		return;
+	}
+
+	wl_list_for_each(child, &sub->surface->subsurface_list, parent_link) {
+		if (child->surface == sub->surface)
+			wl_list_insert(compositor->view_list.prev, &view->link);
+		else
 			view_list_add_subsurface_view(compositor, child, view);
+	}
 }
 
 static void
@@ -1626,11 +1633,18 @@ view_list_add(struct weston_compositor *compositor,
 	struct weston_subsurface *sub;
 
 	weston_view_update_transform(view);
-	wl_list_insert(compositor->view_list.prev, &view->link);
 
-	wl_list_for_each(sub, &view->surface->subsurface_list, parent_link)
+	if (wl_list_empty(&view->surface->subsurface_list)) {
-		if (sub->surface != view->surface)
+		wl_list_insert(compositor->view_list.prev, &view->link);
+		return;
+	}
+
+	wl_list_for_each(sub, &view->surface->subsurface_list, parent_link) {
+		if (sub->surface == view->surface)
+			wl_list_insert(compositor->view_list.prev, &view->link);
+		else
 			view_list_add_subsurface_view(compositor, sub, view);
+	}
 }
 
 static void