refactor(BillboardCollection): Require instancing, remove batch mode

[donmccurdy]

Description

For discussion. Removes fallback code paths from BillboardCollection.js and related code, for cases where ANGLE_instanced_arrays is not available. Support for instancing at 99.99% globally according, and required in WebGL 2:

https://web3dsurvey.com/webgl/extensions/ANGLE_instanced_arrays

That said... there might be reasons to keep the fallback that aren't captured in these stats? It would be a nice simplification in billboard code if possible, and avoids further changes being required in #13050.

Issue number and link

• Fixes Billboards: Require WebGL 1 + ANGLE_instanced_arrays + vertex texture reads (or WebGL 2) #13053

Testing plan

TBD

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

PR Dependency Tree

• PR fix(textureatlas): Allocate texture space more incrementally on resize #13042
  • PR refactor(BillboardCollection): Require instancing, remove batch mode #13049 👈
    • PR fix(BillboardCollection): Fix precision loss in billboard image texcoords #13050

This tree was auto-generated by Charcoal

[github-actions]

Thank you for the pull request, @donmccurdy!

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

[donmccurdy]

Closing this PR for now, but proposing that we add a deprecation warning that billboards will require ANGLE_instanced_arrays and vertex shader texture reads in an upcoming CesiumJS release. See:

• Billboards: Require WebGL 1 + ANGLE_instanced_arrays + vertex texture reads (or WebGL 2) #13053