Change DataSourceDisplay ready state behavior for bounding sphere calculations

[jjspace]

Description

Slightly reverting the change in #12230 and replacing PR #12322 in favor of changing the ready value to fix both #4647 and the newer issue #12303.

The core of this change is to make the DataSourceDisplay._ready boolean to remain true once it's set to true even if the internal datasources become "un-ready" due to transformations or loading.

Issue number and link

Alternative fix for #4647
Fixes #12303

Testing plan

• Test this sandcastle which demonstrates the error when setting the viewer.selectedEntity
• Test this sandcastle to test the original issue of a "flickering" entity when it moves (it should not flicker)
• Test some other sandcastles that pertain to entities and selections to make sure selection and tracking still works as expected

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjspace!

✅ We can confirm we have a CLA on file for you.

[jfayot]

That looks good to me @jjspace ! Having different scratch objects seems so obvious now that I see it... Thanks

[jjspace]

Awesome, thanks for taking a look @jfayot. The primary thing that helped was making sure the ready state stays true because it was flipping between ready and not ready as entities moved which caused the flicker before. The separate scratch variables is more of a sanity thing. They were interacting really weirdly because of how it's passed into and referenced in the EntityView and used in multiple locations that made it very hard to debug.