interval: Transform values on set, if needed

interval: Transform values on set, if needed

It's possible that GValues passed to a ClutterInterval setter are not
of the same type as the interval - for instance, if they come from
language bindings, or from untrusted sources; we can use the same
transformation functions we already use inside ClutterTransition to
ensure that the ClutterInterval always stores values of the same type
used to create the interval itself.