display.c: Exclude 'always on top' windows from the monitor

fullscreen check.

Commit a88891a loosened the rules on what could 'break' fullscreen
mode for a monitor, in order to improve usability in situations
where the user 'alt-tabs' to another application but no panel
appears - there's no way out except back to the fullscreen window.

Unfortunately this broke a use-case of having a small window pinned
above a fullscreen application. So skip these windows in our
calculations.

Fixes linuxmint/cinnamon#13463.

Author: mtwebster

src/core/display.c

@@ -3542,7 +3542,7 @@ check_fullscreen_func (gpointer data)
           if (meta_window_is_monitor_sized (window))
             covers_monitors = TRUE;
         }
-      else if (window->type == META_WINDOW_NORMAL)
+      else if (window->type == META_WINDOW_NORMAL && !window->wm_state_above)
         {
           MetaRectangle window_rect;
           meta_window_get_frame_rect (window, &window_rect);