Fix race condition in TreeSupportBaseCircle which may sometimes cause a crash

[ThomasRahm]

Description

The non-proper locking of base_circle has caused a crash for me in my cradle support branch. I don't know it that crash can also occur here, but either way the lock should actually do something.

• Bug fix (non-breaking change which fixes an issue)

Test Configuration:

• Operating System:

Checklist:

• My code follows the style guidelines of this project as described in UltiMaker Meta
• I have read the Contribution guide
• I have commented my code, particularly in hard-to-understand areas
• I have uploaded any files required to test this change

[github-actions]

Test Results

0 tests   - 27   0 ✅  - 27   0s ⏱️ -5s
0 suites  -  1   0 💤 ± 0 
0 files    -  1   0 ❌ ± 0 

Results for commit 5a74ed4. ± Comparison against base commit ba89f84.

♻️ This comment has been updated with latest results.

[ThomasRahm]

Talked with a friend about this pull request, he suggested to instead use std::call_once as the compiler may reorder the base_circle and boolean assignments, so even my fixed version was not guaranteed to be actually thread-safe. So I changed it to use that instead.
GitHub's diff is over-exaggerating the changes, I recommend to use hide whitespaces to see the actual changes.

[wawanbreton]

Hi @ThomasRahm, I am not familiar with std::call_once, but it seems to do exactly what you need here. There are different ways of achieveing this, but this one is very elegant.