MARLBORO, Vt. — An essential aspect of the Marlboro Music festival is the balance it achieves between the heritage of its illustrious past and the need for renewal and freshness. One of its strategies for achieving this is mixing performers of differing generations and experience levels, ensuring both continuity and variation. Another is a residency in which a prominent composer comes to the festival each summer to coach his or her works. This year it is Polish composer Krzysztof Penderecki, two of whose pieces were on Marlboro programs this past weekend.