Rarely has urban renewal had so much sparkle. On 14 acres of onetime New York City slums, the $1 billion, six-building Lincoln Center for the Performing Arts rose as a high-C cultural capital. To mark its 20th birthday, 1,800 guests (at $250 a ticket) turned out for an al fresco celebration (black tie preferred). Bagpipes skirled, a lighted fountain leaped to the beat of Handel, Copland and Bernstein, and Barbara Walters and Walter Cronkite narrated a history of Lincoln Center via a giant five-section screen (170’x100′) anchored to the facade of the Metropolitan Opera House. Gerald Rafshoon pronounced the center’s image excellent, and Ruth Carter Stapleton thought it a fine spot for a revival. Artist Larry Rivers served up a five-foot-wide cake model of the center, which was not for eating. But between the cocktail buffet, dinner inside two huge halls and post-disco omelets, nobody went home hungry.