message: demo_to_kinects.launch: don't fill empty space
diff: <param name="frame_id" type="string" value="base_link"/>
  <param name="gen_scan" type="bool" value="true"/>
  <param name="wait_for_transform" type="bool" value="$(arg wait_for_transform)"/>
  - <param name="map_negative_poses_ignored" type="bool" value="true"/>
  + <param name="map_negative_poses_ignored" type="bool" value="false"/> <!-- refresh grid map even if we are not moving-->
  + <param name="map_negative_scan_empty_ray_tracing" type="bool" value="false"/> <!-- don't fill empty space between the generated scans-->
  <remap from="rgbd_image0" to="/camera1/rgbd_image"/>
  <remap from="rgbd_image1" to="/camera2/rgbd_image"/>
  