Grease slides into mediocrity at Playhouse Square...by Christine Howey...So it's come to this? A live production of an insanely popular musical needs an actor from the company to come out before the curtain rises to warm up the crowd, like it's a Wheel of Fortune taping? As they say on SNL: Really?! The show is Grease at PlayhouseSquare, and it features well-known, bulletproof material — musical and otherwise — written by Jim Jacobs and Warren Casey. Fortunately, most of the cast can sing just fine (with one idolatrous…...