Fix data race issue in VHACD found by TSAN

[iche033]

🦟 Bug fix

Summary

VHACD can be run multi-threaded, and TSAN picked up a data race issue:

SUMMARY: ThreadSanitizer: data race <path_to_gz-common>/graphics/src/VHACD/VHACD.h:5985:15 in VHACD::VoxelHull::VoxelHull(VHACD::VoxelHull const&, VHACD::SplitAxis, unsigned int)

Changing to an atomic type fixes the problem.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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