Fix visible cable connection for irregular items

The switching station and supply converter only semantically connect to
cables in particular directions.  Make them visually connect only in
the matching directions.  This is done by special-casing in the cable
update logic.  If more irregular items arise in the future, or the
existing items start to need facedir logic, this should be generalised
into something like the connect_sides system for pneumatic tubes.

technic/machines/register/cables.lua

@@ -130,7 +130,11 @@ end
 function technic.cables_should_connect(pos1, pos2, tier)
 	local name = minetest.get_node(pos2).name
 
-	if technic.is_tier_cable(name, tier) then
+	if name == "technic:switching_station" then
+		return pos2.y == pos1.y + 1 and "machine" or false
+	elseif name == "technic:supply_converter" then
+		return math.abs(pos2.y - pos1.y) == 1 and "machine" or false
+	elseif technic.is_tier_cable(name, tier) then
 		return "cable"
 	elseif technic.machines[tier][name] then
 		return "machine"