Added Nigel's suggested code to clear all but the destroy callback early

Added Nigel's suggested code to clear all but the destroy callback early
on, leaving Destroy to be cleared later after the last possible point
where it should be invoked.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@329 7f0cb862-5218-0410-a997-914c9d46530a