Add per client private data, which for now is empty.
This is a preparation patch for adding randr/vidmode resolution
change emulation.
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>