“Few terms in the realms of musical composition and theater are as imprecise, let alone patronizing, as incidental music. … Yet since the 17th century when Purcell wrote ingenious music for English plays, notable composers have written music for the theater that was hardly incidental.” (Grieg’s Peer Gynt and Mendelssohn’s Midsummer Night’s Dream are but two famous examples.) Anthony Tommasini looks at the new scores accompanying this year’s two Shakespeare in the Park (that’s Central Park) productions.