First pass at double-buffered drawing

[Owen Taylor <otaylor redhat com>]

Since there seems to be a strong interest in the idea
of double-bufferingback, I've checked my current working
tree into CVS on the branch gtk-no-flicker. 

So, (allowing for usual anon-cvs propagation delays),
you should be able to get it from there.

 - This code/API is still very much in flux
 - There are no docs, not even a ChangeLog. 
 - Some things don't work even in testgtk, it has not been
   tested with _anything_ other than testgtk.
 - It's completely unoptimized; it creates and destroys
   GC's all over the place, etc.

But it is sort of neat; it certainly opaque-resizes
far, far, better than GTK+ has ever done before.

And in general, the code is much simpler than it was
before; I was able to remove a considerable amount
of the most complex parts of gtkwidget.c by putting the
draw queueing logic into GDK.

Regards,
                                        Owen