Changed var names. Ommitted mid/center-stage. Renamed to start.

Sat with this, and walked through a scenario.  I could try and explain this here, but figure it's probably best to sit down with a sketch and walk through some diagrams.  Mid / centerstage seems to really be the start of the animation.  So I renamed that (back to) start.  All intro takes place during inWindow, that hasn't changed from the original version, it's just that outro seems to occur for the entirety of the scroll past "end" or downStage. Which was surprising for me. I left aboveWindow alone, as I couldn't remember what we were calling it.