patch 8.0.1084: GTK build has compiler warnings

Problem:    GTK build has compiler warnings. (Christian Brabandt)
Solution:   Get screen size with a different function. (Ken Takata, Yasuhiro
            Matsumoto)
diff --git a/src/gui_beval.c b/src/gui_beval.c
index abff1b0..61ff2fc 100644
--- a/src/gui_beval.c
+++ b/src/gui_beval.c
@@ -1177,23 +1177,15 @@
 	int		x_offset = EVAL_OFFSET_X;
 	int		y_offset = EVAL_OFFSET_Y;
 	PangoLayout	*layout;
-# if GTK_CHECK_VERSION(3,22,2)
-	GdkRectangle rect;
-	GdkMonitor * const mon = gdk_display_get_monitor_at_window(
-		gtk_widget_get_display(beval->balloonShell),
-		gtk_widget_get_window(beval->balloonShell));
-	gdk_monitor_get_geometry(mon, &rect);
 
-	screen_w = rect.width;
-	screen_h = rect.height;
-# else
+# if !GTK_CHECK_VERSION(3,22,2)
 	GdkScreen	*screen;
 
 	screen = gtk_widget_get_screen(beval->target);
 	gtk_window_set_screen(GTK_WINDOW(beval->balloonShell), screen);
-	screen_w = gdk_screen_get_width(screen);
-	screen_h = gdk_screen_get_height(screen);
 # endif
+	gui_gtk_get_screen_size_of_win(beval->balloonShell,
+							 &screen_w, &screen_h);
 # if !GTK_CHECK_VERSION(3,0,0)
 	gtk_widget_ensure_style(beval->balloonShell);
 	gtk_widget_ensure_style(beval->balloonLabel);