code: public ImageLayer setTile (RFuture<? extends Tile> tile) {
      tile.onSuccess(new Slot<Tile>() {
        public void onEmit (Tile tile) { setTile(tile); }
      });
      return this;
    }
docs: Sets the tile rendered by this layer to the asynchronous result of {@code tile}. When the
  future completes, this layer's tile will be set. Until then, the current tile (if any) will
  continue to be rendered.