[compiz] window walking interface

[David Reveman]

I had a look at the interface that allows plugins to affect the stacking
of windows during rendering. Just adding first, next, prev, etc.
function pointers that can be wrapped didn't seem very appropriate as it
doesn't make sense to allow plugins to just wrap some of those functions
and the overhead from going through the whole wrapping chain every time
next or prev is called is unnecessary.

I've added a simple window walking interface that should be sufficient.
It should be easy to tell how it works from the changes to screen.c and
paint.c.

We still have to do something about the clip planes, right? I guess a
wrap-able setOutputClipPlanes function will do for now. I'll make this
change next unless you've got a better idea.

It should be fairly easy to properly integrate any 3D window elevation,
translucent cube and sticky window mode now that plugins can hook into
the cube plugins structures. Just add whatever hooks you need to the
cube plugin.

-David