The deferred window destruction code was destroying the windows in reverse

The deferred window destruction code was destroying the windows in reverse
order.  This cased a crash when the call to glutDestroyWindow() for a sub
windows was immediately followed by a call to glutDestroyWindow() for it's
parent. fgCloseWindows() would call fgDestroyWindow() for the parent first
fgDestroyWindws() would recurse over the children and then fgCloseWindows()
would call fgDestroyWindow() again for the child.

I've replaced the single linked list with one of our two way link list
structures.  I've also moved it into fgStructure because that seemed the
consistent thing to do.

I said the the deferred windows destruction causes more problems then it
solves.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@396 7f0cb862-5218-0410-a997-914c9d46530a