tutorials: headless_rendering: Little fixes

[peci1]

🦟 Bug fix

Summary

Automatically run the gz server (-r) so that readers are not surprised by no output from the simulator.

Added DISPLAY= to commands to ensure the code is not using an X server.

Changed gz topic -et to gz topic -ft to not overload the user's console with binary streams.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[j-rivero]

Thanks Martin for the PR. Do you think that would be a good idea to include @iche033 suggestions for checking that EGL is working in this tutorial gazebosim/gazebo_test_cases#1182 (comment) ? Could be a good way of knowing that there is no huge problems without running the camera rendering.

[peci1]

Good idea. Added.

[j-rivero]

Amazing thanks!