Some region selection fixes

- CuboidSelection no longer deals with IncompleteRegionExceptions
- Fixed Polygonal2DSelection not passing its region to the selector
- Fixed Polygonal2DRegion not cloning the list it receives in its constructor
- Gave PolygonalRegionSelector a new constructor where it takes a list of points

src/main/java/com/sk89q/worldedit/regions/Polygonal2DRegion.java

@@ -76,7 +76,7 @@ public class Polygonal2DRegion implements Region {
      * @param maxY
      */
     public Polygonal2DRegion(LocalWorld world, List<BlockVector2D> points, int minY, int maxY) {
-        this.points = points;
+        this.points = new ArrayList<BlockVector2D>(points);
         this.minY = minY;
         this.maxY = maxY;
         hasY = true;