Port "SStechtree startup optimisation" (cmss13-devs#343)

Co-authored-by: SabreML <[email protected]>

code/controllers/subsystem/techtree.dm

@@ -34,17 +34,6 @@ SUBSYSTEM_DEF(techtree)
 		var/datum/space_level/zpos = SSmapping.add_new_zlevel(tree.name, list(ZTRAIT_TECHTREE))
 		tree.zlevel = zpos
 
-		var/zlevel = zpos.z_value
-		var/turf/z_min = locate(1, 1, zlevel)
-		var/turf/z_max = locate(world.maxx, world.maxy, zlevel)
-
-
-
-		for(var/t in block(z_min, z_max))
-			var/turf/Tu = t
-			Tu.ChangeTurf(/turf/closed/void, list(/turf/closed/void))
-			new /area/techtree(Tu)
-
 		for(var/tier in tree.tree_tiers)
 			tree.unlocked_techs += tier
 			tree.all_techs += tier

code/modules/cm_tech/techtree.dm

@@ -58,16 +58,24 @@
 		if(longest_tier < tier_length)
 			longest_tier = tier_length
 
-	// Clear out the area
-	for(var/t in block(locate(1, 1, zlevel.z_value), locate(longest_tier * 2 + 1, length(all_techs) * 3 + 1, zlevel.z_value)))
+	// Clear out and create the area
+	// (The `+ 2` on both of these is 1 for a buffer tile, and 1 for the outer `/turf/closed/void`.)
+	var/area_max_x = longest_tier * 2 + 2
+	var/area_max_y = length(all_techs) * 3 + 2
+	for(var/t in block(locate(1, 1, zlevel.z_value), locate(area_max_x, area_max_y, zlevel.z_value)))
 		var/turf/pos = t
 		for(var/A in pos)
 			qdel(A)
 
-		pos.ChangeTurf(/turf/open/blank)
-		pos.color = "#000000"
-
+		if(pos.x == area_max_x || pos.y == area_max_y)
+			// The turfs around the edge are closed.
+			pos.ChangeTurf(/turf/closed/void)
+		else
+			pos.ChangeTurf(/turf/open/blank)
+			pos.color = "#000000"
+		new /area/techtree(pos)
 
+	// Create the tech nodes
 	var/y_offset = 1
 	for(var/tier in all_techs)
 		var/tier_length = length(all_techs[tier])