[compiz] multi-head changes

[David Reveman]

I've pushed out all ABI changes that I think are needed for future
multi-head support. I wanted to get those things into the 0.2 branch. No
plugins have been updated or added other than some minor things in the
place plugin. I don't plan to do any of this before the 0.2 release but
as the ABI changes are now in there, we can add it to the 0.2 branch
later.

The changes are mostly about getting each head to be rendered separately
so that plugins can easily treat them as individual views your desktop.

-David