def layer(source, &blk)
      layer = SimplerTiles::VectorLayer.new(source, &blk)
      add_vector_layer layer
    end
def layer(source, &blk)
      layer = SimplerTiles::VectorLayer.new(source, &blk)
      add_vector_layer layer
    end
Add a layer to the c list of layers and yield the new layer.