Defer DestroyTileset to the next Tick when RefreshTileset is called

[j9liu]

Description

This implementation is inspired by what we do in Cesium for Unity. Instead of calling DestroyTileset immediately in RefreshTileset, this queues the function to happen on the next Tick(). This does not affect object destruction when the Actor itself is destroyed.

Issue number or link

Fixes #1784.

Author checklist

• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• [ ] I have added or updated unit tests to ensure consistent code coverage as necessary.
• [ ] I have updated the documentation as necessary.

Testing plan

Go through the Reproduction Steps in #1784 and confirm they don't result in a crash.

[j9liu]

I saw that the height sampling tests were failing, and I don't know if I understand the logistics of how the Tick prevented the test from succeeding. But I saw that RefreshTileset was being invoked during ResolveCreditSystem, so I changed it to a direct DestroyTileset call that mimicked the old behavior.

Before that, I moved the _destroyOnNextTick = false line to the top of DestroyTileset, and that seemed to work too. But it seems better to unset that in the actual Tick function.

[j9liu]

I don't know if I understand the logistics of how the Tick prevented the test from succeeding.

Silly me... it's because DestroyTileset calls the Tileset destructor, which immediately fails all height requests! And although ResolveCreditSystem is called right away in SampleHeightMostDetailed, the RefreshTileset won't be processed until the next Tick as expected.

[david-lively]

Looks good. Thanks!