Fix race condition in table initialization with multiple threads

[brechtvl]

This caused intermittent wrong encoding in the OpenEXR integration, which then resulted in "Error decoding a codeblock" while decoding.

Use std::call_once to resolve this.

[brechtvl]

This issue was found integrating the new OpenEXR HTJ2K encoding in Blender, and I confirmed that a randomly failing test now reliably passes.

The bug was found and fixed by Gemini CLI, but I did review the fix carefully. A program to reproduce the issue using thread sanitizer was also generated, I've attached that. The fix resolves all thread sanitizer warnings in that program. I didn't see a way to reproduce this with existing programs included with OpenJPH.
race_condition_repro.zip

Potentially related issues:
AcademySoftwareFoundation/openexr#2207
#223

[aous72]

Dear Brecht,

Thank you for this important discovery and for the PR.

I am well with the suggested changes, but I am allergic to opaque C++ code.
I would like to use std::atomic_flag or std::atomic_bool for the initialization flag, and a simple std::atomic_flag lock to protect the critical section (the initialization section), if that is alright with you. Still not very clear on how to do that.

I am happy to do the changes if that is alright with you.

Thanx again.

Kind regards,
Aous.

[brechtvl]

Thanks for the quickly reply. Feel free to make changes.