Fix 1-frame background glitch when returning from in-game options

The background would change for 1 frame before sending you back to the
pause menu or editor settings. The map.nexttowercolour() call needs to
be deferred until the end of the frame.

desktop_version/src/Input.cpp

@@ -1638,10 +1638,9 @@ void titleinput(void)
                 else
                 {
                     game.returnmenu();
+                    map.nexttowercolour();
                 }
             }
-
-            map.nexttowercolour();
         }
 
         if(game.menustart)