Fixed broken X11 menus. The MPX patch would naively just call the

standard (non-multi) callbacks directly, overriding all the
menu-handling logic in the standard X event processing loop.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1375 7f0cb862-5218-0410-a997-914c9d46530a