clear window workmask when queuing up window for destruction. All callbacks (except...

clear window workmask when queuing up window for destruction. All callbacks (except destroy) are removed from the window already anyway. This fixes a problem where a newly created window that is immediately destroyed again causes an error-exit because it is still being inited and at that point it is found that no display callback has been set, which is an error.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1837 7f0cb862-5218-0410-a997-914c9d46530a