Fix crash with switching terrain provider and picking

[hpinkos]

Fixes #6916

The crash was caused when things happen in the following order:

1. When a new terrain provider is set, QuadtreePrimitive._invalidateAllTiles is set to true
2. scene.render is called, which queues commands, including the commands for the soon-to-be invalidated GlobeSurfaceTiles. QuadtreePrimitive actually invalidates these tiles at the end of the frame.
3. scene.pick is called. It uses the same commands from the last render, which now includes commands for tiles that were freed at the end of the frame. This causes a crash because the vertex arrays for these commands have been destroyed.

Moving invalidateAllTiles to frameStart instead of frameEnd prevents the commands for the invalid tiles from being added to the command queue.

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

---

I am a bot who helps you make Cesium awesome! Contributions to my configuration are welcome.

🌍 🌎 🌏

[bagnell]

Update CHANGES.md.

Your reason for the crash and the code changes look good to me, but I couldn't reproduce the crash in master. Can someone else confirm this fixed the issue? @lilleyse @mramato?

[hpinkos]

@bagnell yeah it's really difficult to reproduce with the code sample I included in #6916 but I can reproduce it easily in another project if you want to test that out

[bagnell]

OK, I can confirm this fixes the issue. Thanks @hpinkos.