Ensures area contents respects world.maxx/y expanding (#72283)

## About The Pull Request

It never like, happens, and if it ever did we'd yell at the guy who did
it
But if we're gonna pretend we support expanding in the maploader, we
should support it structurally too

code/modules/mapping/reader.dm

@@ -319,8 +319,12 @@
 	var/relative_y = first_column.ycrd
 	var/highest_y = relative_y + y_relative_to_absolute
 
-	if(!cropMap && highest_y > world.maxy)
-		world.maxy = highest_y // Expand Y here. X is expanded later on
+	if(!cropMap && highest_y > world.maxx)
+		if(new_z)
+			// Need to avoid improperly loaded area/turf_contents
+			world.increaseMaxY(highest_y, max_zs_to_load = z_offset - 1)
+		else
+			world.increaseMaxY(highest_y)
 		expanded_y = TRUE
 
 	// Skip Y coords that are above the smallest of the three params
@@ -353,7 +357,11 @@
 		var/delta = max(final_x - x_delta_with, 0)
 		final_x -= delta
 	if(final_x > world.maxx && !cropMap)
-		world.maxx = final_x
+		if(new_z)
+			// Need to avoid improperly loaded area/turf_contents
+			world.increaseMaxX(final_x, max_zs_to_load = z_offset - 1)
+		else
+			world.increaseMaxX(final_x)
 		expanded_x = TRUE
 
 	var/lowest_x = max(x_lower, 1 - x_relative_to_absolute)
@@ -443,7 +451,11 @@
 		var/ycrd = relative_y + y_relative_to_absolute
 		var/zcrd = gset.zcrd + z_offset - 1
 		if(!cropMap && ycrd > world.maxy)
-			world.maxy = ycrd // Expand Y here.  X is expanded in the loop below
+			if(new_z)
+				// Need to avoid improperly loaded area/turf_contents
+				world.increaseMaxY(ycrd, max_zs_to_load = z_offset - 1)
+			else
+				world.increaseMaxY(ycrd)
 			expanded_y = TRUE
 		var/zexpansion = zcrd > world.maxz
 		var/no_afterchange = no_changeturf
@@ -495,7 +507,11 @@
 			final_x -= delta
 			x_target = x_step_count * key_len
 		if(final_x > world.maxx && !cropMap)
-			world.maxx = final_x
+			if(new_z)
+				// Need to avoid improperly loaded area/turf_contents
+				world.increaseMaxX(final_x, max_zs_to_load = z_offset - 1)
+			else
+				world.increaseMaxX(final_x)
 			expanded_x = TRUE
 
 		// We're gonna track the first and last pairs of coords we find