Tweak vision of walls and some special structures.

Those buildings get 1 vision (making them selectable), but no longer provide vision on their own.
Wall Towers have their vision increased slightly to make it larger than their attack range and to compensate for it.

Patch by: Nescio

Accepted By: borg-, genava55, wraitii

Differential Revision: https://code.wildfiregames.com/D2505