xdg-shell: restrict or clarify behavior with negative width and height in configure.

xdg_toplevel.configure does not explicitly forbid a compositor from sending a width and or height with a negative value. Is this something a compositor could do or is this a case where xdg-shell is under-specified. I don't see any real good reason to have negative width or height.

It's also unclear how clients may behave if negative width and height values are sent in a configure. I'd think it would be best to forbid negative width and height in a new version of xdg-shell.

This might also apply in some other shell protocols, but those can be addressed in other issues.