This should fix Bug#31223, Bug#28106, Bug#23672 as well as Ubuntu bug
https://bugs.launchpad.net/ubuntu/+source/emacs25/+bug/1695228

Also fixes the formerly unscaled Y value returned by
frame-monitor-workarea (and display-monitor-attributes-list).

For details on why some GTK menus were empty please see thread
https://lists.gnu.org/archive/html/emacs-devel/2019-11/msg01061.html

* src/gtkutil.c
   (menubar_map_cb): properly scale req.height so that the menu bar's
   height is in device pixels as expected
   (xg_update_frame_menubar): dito
   (xg_event_is_for_menubar): properly scale rec.x and rec.y so that
   gtk_widget_intersect() works as intended
* src/xfns.c
   (Fx_display_monitor_attributes_list): properly scale work.x and work.y