Walls can be constructed on top of existing walls

[gudo]

Steps to recreate:

*Build a wall segment long enough for a gate
*Turn said segment into a gate
*Attempt to build a wall over the gate (re-build the exact same segment)

*It is impossible to build other structures inside the gate.
*This will work regardles of wether or not the gate is locked.
*This will only work when you try to rebuild the exact same segment that you turned into a gate. (You cannot have a new wall bisect a gate)

[Deiz]

In 12269:

Added a function for finding colliding entities with identical control groups. Refs #1529.

[Deiz]

In 12270:

Do a second check when trying to place wall pieces with two control groups. Fixes #1529.

[Deiz]

The root cause of this is unrelated to gates; it actually occurred because the existing wall piece (or gate) would have two control groups, allowing it to collide with towers on either end. Placing a new wall between those two towers would give the new wall the same control groups, thereby allowing it to collide with the gate, too.