Robert Rauschenberg was not a giant of American art; he was the giant. No American created so many esthetic openings for so many artists. Jasper Johns, his sometime lover, said, "Rauschenberg was the man who in this century invented the most since Picasso." His output always bordered on the mad and ecstatic; his art could be theatrical, wan, redundant or just cruddy-looking. In fact, everything he made, good or bad -- and many think his late work is junk -- has an edge of wit, optical nerve and invention.