[GtkGLExt] subwindows for newby

[ivan danicic <ivand blueyonder co uk>]

Hello All, in order to achieve the equivalent of glutCreateSubwindow (which 
look just like plain rectangles) I think I need to be able to make a 
gtk_window without any borders, decorations, etc. How can I do that?
I need to do this because I want the subwindow to have its own projection 
matrix (and stack) but to be non-removable and in fact looking just like a 
drawing_area. Any suggestions gratefully anticipated.
Ivan