“Gladiator II” is a spectacle, but is it any good? That’s the question the two-and-a-half-hour extravaganza poses to even the most ardent fan of violence and Virgil. The first installment, which won at the box office and the Oscars, could credibly claim Julius Caesar’s motto — veni, vidi, vici, “I came, I saw, I conquered.” Now, its octogenarian director, Ridley Scott — few Caesars lived so long — is back in the Colosseum, hoping for a second upturned thumb.