Add a flag to force headless rendering mode

[ahcorde]

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🎉 New feature

Summary

This PR depends on gazebosim/gz-rendering#272. This will allow to use the flag --headless to force ignition-rendering to use the headless mode if it's available.

Test it

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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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[codecov]

Codecov Report

Merging #701 (bea1038) into main (f0e235b) will decrease coverage by 0.01%.
The diff coverage is 91.30%.

❗ Current head bea1038 differs from pull request most recent head 8b96f3e. Consider uploading reports for the commit 8b96f3e to get more accurate results

@@            Coverage Diff             @@
##             main     #701      +/-   ##
==========================================
- Coverage   63.28%   63.27%   -0.02%     
==========================================
  Files         238      239       +1     
  Lines       19423    19445      +22     
==========================================
+ Hits        12292    12303      +11     
- Misses       7131     7142      +11     

Impacted Files	Coverage Δ
include/ignition/gazebo/ServerConfig.hh	100.00% <ø> (ø)
include/ignition/gazebo/rendering/RenderUtil.hh	100.00% <ø> (ø)
src/rendering/RenderUtil.cc	36.49% <71.42%> (+0.17%)	⬆️
...on/gazebo/components/RenderEngineServerHeadless.hh	100.00% <100.00%> (ø)
src/LevelManager.cc	89.03% <100.00%> (+0.08%)	⬆️
src/ServerConfig.cc	90.33% <100.00%> (+0.15%)	⬆️
src/ign.cc	72.36% <100.00%> (+0.18%)	⬆️
src/systems/sensors/Sensors.cc	71.74% <100.00%> (+0.51%)	⬆️
...int_position_controller/JointPositionController.cc	53.14% <0.00%> (-7.43%)	⬇️
... and 2 more

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[chapulina]

I think the word "headless" can be confusing to Gazebo users. We often use that word to refer to a server-only simulation, without the GUI process.

Maybe headless-rendering?

[ahcorde]

@mjcarroll @chapulina open for review

[chapulina]

I just have some minor comments, but the changes LGTM. I haven't tried actually running it headless with the upstream PR though.

I think it would be best to merge this only after gazebosim/gz-rendering#272 gets in, so the functionality is working.

[chapulina]

I just tried the flag and it works for me, but I haven't tried it in a container without access to the display.