Implement getMaxY for ForgeWorld.

Should play nicer with things like CubicChunks. Note that there is no
way to get the min point in Minecraft itself, so this only supports
going up, not down.

worldedit-forge/src/main/java/com/sk89q/worldedit/forge/ForgeWorld.java

@@ -426,6 +426,11 @@ public class ForgeWorld extends AbstractWorld {
         }
     }
 
+    @Override
+    public int getMaxY() {
+        return getWorld().getHeight();
+    }
+
     @Override
     public BlockVector3 getSpawnPosition() {
         return ForgeAdapter.adapt(getWorld().getSpawnPoint());