message: rtabmap.launch: set approx_sync=false by default is not subscribing to any camera topics
diff: <!-- if timestamps of the input topics are synchronized using approximate or exact time policy-->
  <arg if="$(arg stereo)" name="approx_sync" default="false"/>
  - <arg unless="$(arg stereo)" name="approx_sync" default="true"/>
  + <arg unless="$(arg stereo)" name="approx_sync" default="$(arg depth)"/>
  <!-- RGB-D related topics -->
  <arg name="rgb_topic" default="/camera/rgb/image_rect_color" />
  