Implemented physics collision for MeshRoad

According to [deepscratch's post](http://www.garagegames.com/community/forums/viewthread/130181/1#comment-826897) in the forum. It seems like his fix had been half-implemented, adding `mPhysicsRep` to `MeshRoad`, but it was still unused. This adds the remainder of his code, as well as an extra line that deletes the existing `mPhysicsRep` before creating a new one.