P_CheckPosition optimization

[Major Cooke]

Pull Request

A small code change. I noticed that the THRUACTORS flag is being needlessly checked throughout PIT_CheckThing for the actor that calls (P_)CheckPosition, so I modified it to not go through the actor checking loop if the actor passed through has the flag.

Also pushed the THRUACTORS flag check up higher over the distance check in PIT_CheckThing itself.