Fix hollowOutRegion mutating vector

Fixes #3083

Author: eztaK-red

worldedit-core/src/main/java/com/sk89q/worldedit/EditSession.java

@@ -3298,7 +3298,7 @@ public int hollowOutRegion(Region region, int thickness, Pattern pattern, Mask m
                 outer:
                 for (BlockVector3 position : region) {
                     for (BlockVector3 recurseDirection : recurseDirections) {
-                        BlockVector3 neighbor = position.add(recurseDirection);
+                        BlockVector3 neighbor = position.toImmutable().add(recurseDirection);
 
                         if (outside.contains(neighbor)) {
                             newOutside.add(position);
@@ -3313,7 +3313,7 @@ public int hollowOutRegion(Region region, int thickness, Pattern pattern, Mask m
             outer:
             for (BlockVector3 position : region) {
                 for (BlockVector3 recurseDirection : recurseDirections) {
-                    BlockVector3 neighbor = position.add(recurseDirection);
+                    BlockVector3 neighbor = position.toImmutable().add(recurseDirection);
 
                     if (outside.contains(neighbor)) {
                         continue outer;