Meanwhile, in a more urbane, Rockefeller-meets-DuPont setting, the Winterthur in Delaware hosts 86 splendid paintings from San Francisco's de Young Museum, half of them donated by John D. Rockefeller III. ''American Accents, 1670-1945: Masterworks From the Fine Arts Museums of San Francisco,'' runs through Feb. 1 and features artists such as John Singer Sargent, John Singleton Copley, Thomas Eakin