ClutterInterval.compute_value() computes the new value given a progress
and copies it to a given GValue. Since most of the time we want to pass
that very same value to another function that copies it again, we should
have a compute_value() variant that stores that computed value inside
ClutterInterval and returns a pointer to it. This way we initialize the
result GValue just once and we never copy it, as long as the Interval
instance is valid.