survey function now uses Vol's new voxel exposure function

cover AI was completely broken because I reversed the signs between civilians and aliens... I really globbed up
also broken was the distance squared function because I must have been asleep .___.

also made units with no TUs stop calculating tile scores, duh. This is especially relevant to reapers.

Also, on Vol's suggestion, just restricted coverAI to searching tiles within the findReachable() set; it feels way faster now!

Also converted the big for() loop in the screenshot function into one SDL_BlitSurface() call.