Fix a 100% CPU usage on storyscreens without text. Reported by Soliton.

changelog

@@ -11,6 +11,7 @@ Version 1.7.3+svn:
    * Removed obsolete code for implicit linked widgets for the listbox
    * [part] caption= is no longer supported; prepend the CAPTION macro
      to story text instead
+   * Fixed 100% CPU usage on storyscreens with no text (e.g. map screens)
  * WML Engine:
    * Support for [show_if] inside [message]	
    * Fix ODR issues in gui2

src/storyscreen/render.cpp

@@ -521,6 +521,7 @@ void part_ui::wait_for_input()
 		events::raise_process_event();
 		events::raise_draw_event();
 		disp_.flip();
+		disp_.delay(20);
 	}
 }