This dichotomy — a fallen society and its insatiable need for entertainment — is the clever and not altogether flattering backdrop of the “Gladiator” films. Part two, set 20 years after the events of the first movie, brings a new combatant to the Coliseum — a mysterious outsider named Lucius Verus, played by Paul Mescal. And to answer the inevitable question, yes. Yes, I was quite entertained.