Keep track of the output the surface enters/leaves.

This is fairly basic tracking though, we do not keep a full list of
outputs a surface may be covering partially, we just keep the output
the surface entered last.

This is sufficient as a preparation work for fullscreen though.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>