Make isTilePresent work correctly

WorldPainter/WPCore/src/main/java/org/pepsoft/worldpainter/RODelegatingDimension.java

@@ -94,6 +94,11 @@ public abstract class RODelegatingDimension<T extends Tile> extends Dimension {
         return dimension.getTileFactory();
     }
 
+    @Override
+    public boolean isTilePresent(int x, int y) {
+        return getTile(x, y) != null;
+    }
+
     @Override
     public int getTileCount() {
         return dimension.getTileCount();