Starting from
http://answers.unity3d.com/users/2664/davea.html
I (alain) jumped there by hazard :
http://answers.unity3d.com/questions/17676/guiwindow-resize-window.html
And tried the script ( OK , not from Dave, but to show what is "to share" :)
http://mixed3d.free.fr/dave/davemixedwindowPlayer.html