Support rendering for all glTF primitive modes

[j9liu]

Description

Depends on #701 so merge that first.

Depends on CesiumGS/cesium-native#1393.

This PR incorporates the above cesium-native PR to render all primitive modes in Unity. Since not all modes are supported out-of-the-box, this uses options in LoadTileset to make cesium-native retopologize as needed.

Mode	Expected
LINE_LOOP
LINE_STRIP
TRIANGLE_STRIP
TRIANGLE_FAN

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• [ ] I have added or updated unit tests to ensure consistent code coverage as necessary.
• [ ] I have updated the documentation as necessary.

Testing plan

1. Start from 01_CesiumWorld
2. Create a new empty game object with a CesiumGeoreference component.
3. Set the georeference to True Origin
4. Add a Blank 3D Tileset and load MeshPrimitiveModes.zip.
5. Also be sure to test the models in primitive-restart-samples.zip for primitive restart support. Edit the tileset.json to change the URL of the referenced .glb.

Confirm that the model matches the above appearance.

Reviewer checklist

Thank you for taking the time to review this PR. By approving a PR you are taking as much responsibility for these changes as the author.

As you review, please go through the checklist below:

• Review and run all parts of the test plan on this branch and verify it matches expectations.
  • If the issue is a bug please make sure you can reproduce the bug in the main branch and then checkout this branch to make sure it actually solved the issue.
• Review the code and make sure you do not have any remaining questions or concerns. You should understand the code change and the chosen approach. If you are not confident or have doubts about the code, please do not hesitate to ask questions.
  • Code should follow the C++ Style Guide
• Review the unit tests and make sure there are no missing tests or edge cases.
• Review documentation changes and updates to CHANGES.md to make sure they accurately cover the work in this PR.
• Verify that the Contributor License Agreement has been submitted, if needed.

[j9liu]

Updated for CesiumGS/cesium-native#1393; turns out that Unity doesn't like when we pass in an index buffer with the primitive restart value (it crashes at MeshData::SetSubMesh), so I avoided using the LineStrip topology entirely.

This is ready for review!

[azrogers]

Thanks @j9liu!