Vector Tiles: Support 3DTILES_content_gltf_vector and EXT_mesh_polygon

[donmccurdy]

Description

Updating CesiumJS support for vector tiles to match the current draft specifications.

3D Tiles extensions

• Add 3DTILES_content_gltf_vector 3d-tiles#838

glTF extensions

• Add KHR_mesh_primitive_restart KhronosGroup/glTF#2569
• Add EXT_mesh_polygon KhronosGroup/glTF#2570

Overview

• Vector data... in glTF? 3d-tiles#825 (comment)

For now I've kept backward compatibility with earlier (internal) drafts, e.g. CESIUM_mesh_vector, but I expect we'll drop that support after the 1.142 release.

Issue number and link

• Vector data... in glTF? 3d-tiles#825

Testing plan

Updated unit tests, sample data, and sandcastle demo. Any existing datasets and sandcastles should remove unaffected and backward-compatible, until support for CESIUM_mesh_vector (which is not part of the current/upcoming Vector Tiles direction) is removed after v1.142.

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

AI acknowledgment

• I used AI to generate content in this PR
• If yes, I have reviewed the AI-generated content before submitting

If yes, I used the following Tools(s) and/or Service(s):

If yes, I used the following Model(s):

[github-actions]

Thank you for the pull request, @donmccurdy!

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

[donmccurdy]

@danielzhong this is ready for another review, but (if the PR otherwise looks ready to you) please do not merge this one yet, I would like to double-check on the release timing with the tilers.