func surfaceGetID(s *C.cairo_surface_t) id {
p := C.cairo_surface_get_user_data(s, idkey)
if p == nil {
panic("surface does not have ID - created outside of cairo binding and not registered")
}
return id(ctoint(p))
}
func surfaceSetID(s *C.cairo_surface_t) {
if C.cairo_surface_get_user_data(s, idkey) != nil {
return
}
C.cairo_surface_set_user_data(s, idkey, generateID(), free)
}