x11: Use XC_top_left_corner/XC_top_right_corner instead of XC_fleur. On Gnome (and hopefully others!), this produces something that actually matches SDL_SYSTEM_CURSOR_SIZENWSE/SDL_SYSTEM_CURSOR_SIZENESW. On other desktop enviroments, it probably fits the spirit better than XC_fleur in any case. Reference Issue #2123.