For its much-ballyhooed Western Maverick (due May 20), Warner Bros. is playing with a full deck-literally. As a promotional gimmick, the studio has been sending out boxes of Maverick playing cards, which feature the film’s tony stars-Jodie Foster, James Garner, and Mel Gibson-as the queens, kings, and jacks. Curiously, however, it’s Garner-and not box office ace Gibson-who’s been dubbed king. Was this a deliberate homage to the Emmy-winning actor who created the original Bret Maverick in the ’50s TV series? ”It was just random,” says a studio spokeswoman. ”They didn’t do any market research.” Maybe it was just a case of age before beauty-or a way to keep the joker wild.