Returns the #GSequenceIter which is @delta positions away from @iter.
If @iter is closer than -@delta positions to the beginning of the sequence,
the begin iterator is returned. If @iter is closer than @delta positions
to the end of the sequence, the end iterator is returned.
Returns the #GSequenceIter which is @delta positions away from @iter. If @iter is closer than -@delta positions to the beginning of the sequence, the begin iterator is returned. If @iter is closer than @delta positions to the end of the sequence, the end iterator is returned.