HORTON FOOTE, a special lecturer tonight at the University of Texas at Arlington, at age 90 is still one of the most vigorous writers in America. He is working on his sixty-first play. "The Trip to Bountiful" played on Broadway and on television fifty years ago. "The Trip to Bountiful" and "Tender Mercies" are his best known works, but he got his first Oscar for the screenplay for "To Kill a Mocking Bird." He acknowledges his debt to William Faulkner and Tennessee Williams, writers whom he knew personally.