Don't redraw lucid menus more than needed. Use double buffer to reduce flicker.

Don't redraw lucid menus more than needed.  Use double buffer to reduce flicker.

* xlwmenu.c: Include Shell.h, remove duplicate declaration of
XlwMenuRedisplay.
(display_menu_item): Replace ws->window with ws->pixmap, remove
call to XftDrawRect.
(display_menu): Remove this and that argument.  Remove
just_compute_this_one_p. Fill pixmap at start and copy it to window
at end.
(expose_cb): New function.
(make_windows_if_needed): Replace XCreateWindow with XtCreatePopup.
Add eventhandler for expose to expose_cb.  Remove creation of
xft_draw.
(create_pixmap_for_menu): New function.
(remap_menubar): Pop down menus that aren't the same as in old_stack.
Set width, heigh, x, y on widget with XtVaSetValues.  Call
create_pixmap_for_menu.
Replace XUnmapWindow with XtPopdown.
Remowe two last parameters to display_menu.
(map_event_to_widget_value, XlwMenuRedisplay, Key, Select)
(pop_up_menu): Remowe two last parameters to display_menu.
(XlwMenuRealize): Call create_pixmap_for_menu, set w and pixmap.
Remove call to XftDrawCreate.
(XlwMenuDestroy): Free pixmap.  Call XtDestroyWidget instead of
XDestroyWindow.
(handle_motion_event): Only call handle_single_motion_event once.

* xlwmenuP.h (window_state): Add pixmap and w.