[RP] preliminary multi-screen support

[shawn]

I've just committed some multi-screen support which I'd half written a
couple weeks ago. With a little help from Mike's patch, I've put
something together. I only have Xnest to test it on, so the behaviour
might not be what you'd want it to be on a REAL multi-screen
setup. Take a look, bust it, post bugs, etc.

BTW, I haven't forgotten Mike's phat patch..its comin'...