Added mutex to protect stored time variables

[sauk2]

🦟 Bug fix

Fixes #2289

Summary

Added a mutex to protect variables simTime and nextUpdateTime as they are used by both render and post update threads.

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)

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