Fix background not changing when pressing Esc on main menu

Followup to #664: Pressing "quit game" instead of using Esc changes the
background, but pressing Esc doesn't. Whoops.

desktop_version/src/Input.cpp

@@ -1603,6 +1603,7 @@ void titleinput(void)
             if (game.currentmenuname == Menu::mainmenu)
             {
                 game.createmenu(Menu::youwannaquit);
+                map.nexttowercolour();
             }
             else
             {