code: public int getTileId(int x, int y, int layerIndex) {
  		Layer layer = (Layer) layers.get(layerIndex);
  		return layer.getTileID(x, y);
  	}
docs: Get the global ID of a tile at specified location in the map
  
  @param x
  The x location of the tile
  @param y
  The y location of the tile
  @param layerIndex
  The index of the layer to retireve the tile from
  @return The global ID of the tile