On a freezing night in December, a bundled-up young crowd filed onto benches in Mast Chocolate's factory at the Brooklyn Navy Yard. They drank wine and ate candy while a 32-piece orchestra sounded the opening notes of Verdi's "Macbeth," presented by the three-year-old LoftOpera.Though the mood was relaxed - toppled beer bottles clanged and a DJ played during intermissions - the actors were no amateurs.