Moved the idle callback out of glutMainLoopEvent() and into

Moved the idle callback out of glutMainLoopEvent() and into
glutMainLoop(), per discussion on the list.  The code looks okay,
to me, though I remain less than wholly convinced that treating idles
as special cases is the best way to go.  Still, a case can be made
for taking "non-event" idle callbacks out of the "event loop".
From an organizational perspective, I think that it's an improvement,
and it fixes a bug for at least one program.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@244 7f0cb862-5218-0410-a997-914c9d46530a