Oh, Bobby D., you used ta be somebody.

Byline: Mark Rahner

What the !@#$% happened to Robert De Niro?

The DVD releases of "Raging Bull" (MGM, 1980, R) and "Shark Tale" (DreamWorks, PG) _ not to mention recent stinkers like "Godsend," "Hide and Seek" and his American Express commercial _ show his slide from film's greatest actor to its most shameless prostitute. Does he have gambling debts? A drunken agent? Or is it like comedian Bobcat Goldthwait once snapped to his critical daughter: "Hey, that talking horse movie paid for this pool."

The two-disc "Raging Bull" special edition delves thoroughly into what many consider to be the best film of the `80s. De Niro won the best actor Oscar in director Martin Scorsese's gorgeous black-and-white story of psychotically churlish boxer Jake La Motta's rise and fall _ physically morphing from a lean and muscular young fighter to a doughy middle-age strip-club MC.

There are three separate …