code: public void appendDirtySprite (Sprite sprite, int tx, int ty)
      {
          DirtyItem item = getDirtyItem();
          item.init(sprite, tx, ty);
          _items.add(item);
      }
docs: Appends the dirty sprite at the given coordinates to the dirty item list.
  
  @param sprite the dirty sprite itself.
  @param tx the sprite's x tile position.
  @param ty the sprite's y tile position.