Suggestions for properly abstracting multiple screens.

[Maarten Maathuis]

Currently multiple screens are not handled in the prettiest way. I
have some suggestions how it could handled in a sane way.

Create a root window for every physical screen, with it's own
framebuffer. In normal situations use that window and expose that to
applications.

Allow "fake" windows to superimposed over multiple root windows, this
could be used if a user wants to treat two screens as one.

Some "rules":

- Don't try hacks like xinerama.
- Only expose real screens to rendering backends.
- Use screen coordinates in most of the server, only using the "fake"
windows when deciding how to clip pictures, when communicating with
clients, etc.

Any thoughts on this?