Desired behavior

We have a custom sensor example here : https://github.com/gazebosim/gz-sim/tree/ign-gazebo6/examples/plugin/custom_sensor_system , and it would be nice to have a custom rendering sensor example as well, that people could use as a template to write their own rendering sensors.

Implementation suggestion

We could leverage #1475, and use osrf/lrauv#213 and a guide. A start could be a camera sensor modified in a trivial way, like mirroring the image.