Use SafeNumeric to protect max_index in ImportThreadData

[jsjtxietian]

When trying to debug #85237 , @TheDuriel and I found a issue in threaded reimport function as the title says. It still did not solve the problem because it's not related to the "correctness" of importing, it affects the progress bar UI.

The real cause of #85237 still needs more investigation.

[AThousandShips]

The thread safety procedure itself looks good to me, not familiar with the larger structure so will need some more eyes on it

[RandomShaper]

I think this is a good use case for SafeNumeric and its exchange_if_greater() function.

[API-Beast]

@RandomShaper
^ OP pushed the requested change.

[lyuma]

Looks good to me. This avoids a data race in the progress bar.

(and generally speaking, data races can lead to a wide range of undefined behavior which could be far outside the scope of just the progress bar, so it is good to fix this)

The change was implemented: This code now uses SafeNumeric<> and exchange_if_greater as requested.

[lyuma]

I would rather merge this rather than the omnibus PR #88122 because it avoids getting bogged down by finding all the people to review bugfixes in three completely separate parts of the engine... and the StringName one risks a serious performance impact.
@RandomShaper if you would like to review the SafeNumeric part, but I think it's pretty straightforward

[RandomShaper]

Being here again, after quite a long time, I realize this could be more accurate (and simpler), by, instead of keeping track of the highest index processed, just having an atomic counter and incrementing it by one from each thread.

The current approach is over-optimistic in that, if item 10 has started processing, 0-9 are assumed to have started, despite scheduling may have caused 10 to start before, say, 5.

Also, shouldn't the counter, either in this PR or in my suggestion, be incremented after processing the item?

This PR already improves things, but giving this as food for thought and possible incremental enhancement.

[akien-mga]

Thanks!