[compiz] Final version of placement enhancement

[Bellegarde Cedric]

David Reveman wrote:
> Thanks,
>
> you were not returning TRUE after successfully updating the mode option

I'm stupid, it was corrected, don't know why i don't get it in final version.

> and I didn't like the way you used a macro to get the work area in each
> placement function. 

Yes, it was bad, now with your commit, i know understand how multihead works.

Cedric