Support disabling threading for rendering sensors about gz-sim

[osrf-migration]

Original comment by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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As @azeey mentioned on the pull request, the option should only disable threading for rendering sensors, within the sensors system. Post-update threading is not affecting determinism.

For more historic context, synchronization between sensors and physics is an old issue on Gazebo classic.

[osrf-migration]

Original comment by Nate Koenig (Bitbucket: Nathan Koenig).

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We could support turning off threading via a tag in a system <plugin>.

I think determinism could be affected by the post-update threading model. Every iteration could have the order of system post-update steps change. Results, not necessary physics or sensor results, could change depending on what is executed in the post-update step. I don't think anything we have now falls into this category, but a downstream user could create their own system that could trigger other processes.

[osrf-migration]

Original comment by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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Ok, so I think it's fair to say that determinism is guaranteed just from the ECM's point of view, since no components or entities can be modified during post-update.