Don't snap when non-belt entities are in both directions.

[Janzert]

aAs posted on mod forum there is a problem when snapping and there are entities in both directions. Currently whichever entity happens to come first in the found list is snapped to.

This changes to an explicit check for entities in both directions and keeps the placed direction if that is found.

One other thing I did notice while testing though is that it will currently snap to the back of underground belts. While it seems completely useless to place a miniloader behind an underground entrance, not snapping does allow a miniloader to place items onto an underground exit. [EDIT: Oops, my initial test was flawed. Not snapping also allows taking off of an underground entrance]