OpenGLUT - window
Even if a window is ``open'', it need not be visible. It may be convenient to hide a window rather than to close it, if you want to re-display the window at the same location and size, later. Redefining all of the OpenGLUT features of a window and adding its
to your tracking when re-opening a window may also be bothersome. So, rather than destroying it, you can simply ask for it to be hidden.