Allow normals to be used when model material is unlit

[lilleyse]

When a model uses the KHR_materials_unlit extension its normal shader attribute is disabled which could be a problem if normals are used for purposes other than lighting. This PR has two fixes:

• Allow normals to still be used but disable lighting code when the material is unlit
• Prevent attributes from being optimized out of the vertex array in case subsequent shader compiles use the normal attribute

Thanks @ErixenCruz and @jtorresfabra for looking into this

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ 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.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

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

[jtorresfabra]

👍 Approved, changes look sensible.
As a proposal maybe in the future we can revisit this part to try to avoid extra shader compilation.

[cesium-concierge]

Thanks again for your contribution @lilleyse!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.

[OmarShehata]

I'm going to merge since this was approved by Jordi and I can confirm it fixes the issue with a non-public test case.