glut: actually manage subwindows
* figl/glut.scm (%make-window): No longer memoized in favour of
manually keeping a table of created windows. Dropped live?
argument. When managing subwindows, the high-level interface no
longer support windows created with the low-level bindings.
(lookup-window): New procedure. Clears up some logic when expecting
to find an existing window.
(current-window, window-parent): Use lookup-window.
(make-subwindow): Renamed from make-sub-window to use the exact term
formally defined and more commonly used in the specification.
(window): New field subwindows.
(destroy-window): Recursively mark subwindows.