This year marks my fourth tour of duty at CinemaCon, and I still can't tell whether it's a blessing or a curse to present first, but STXfilms stepped up to the plate on Tuesday morning and I'd say they hit a solid double. This year feels like a crucial one for STX, which is poised to take a major leap thanks to what appears to be the best slate of films in its short history. The company is already off to a hot start due to the success of The Upside, which has grossed $107 million at …