#4 user sub-window / better sub-window managment

[David Reversat]

When you want to make an interface such as example 5,
you see that you have to create 2 glui subwindow (right
and bottom) this leeds to create 2 glut sub windows
which mean 2 separate openGL context.

I think it will be nicer to have only one glui subwindow
for the UI and another glut subwindow on which the user
can draw.

I'm not sure I remember well how glut work (and if this
possible... ) but I think it will be better this way

David