Francis Ford Coppola’s “Megalopolis” has been in the works for decades, but it feels like it is addressed to an incomprehensible future or a remote past. It landed at the Cannes Film Festival in May, and since then its word of mouth record has been mixed — at best. Half the vanity project of an attenuated magus and half the valedictory of a lion in winter, it reimagines New York as New Rome and comes alive in considering how empires die.