Vertex pathfinder: add the domain edges back to improve behaviour (reverts rP17224)

This adds back the "domain" edges to the short/vertex pathfinder.
Without these edges, units could use points far away from the path search that they inaccurately thought were reachable, leading to some pathing oddities in rare cases.

Reviewed By: temple

Differential Revision: https://code.wildfiregames.com/D443