Add height reference (clamp-to-ground) to ModelExperimental

[j9liu]

This PR adds a heightReference option to ModelExperimental to allow for clamp-to-ground behavior. Most of the code is taken from Model, though ModelExperimental.update had to be reorganized so things could be updated in the right order (model matrix, bounding spheres, reference matrices).

Sandcastle for ModelExperimental, and a sandcastle for Model for comparison in behavior.

I tried to add rendering tests for this but it was difficult to because of the globe. Even if the Model is positioned very high, the globe still peeks over the horizon. Then, globe.show to false disables the clamp-to-ground behavior, so I can't prevent it from rendering during the unit test.

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ 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.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ptrgags]

@j9liu did a pass through the code, looks sensible though I had an idea for refactoring ModelExperimental.update().

I still need to try out the code in Sandcastle, I'll do that next

[ptrgags]

tried various sandcastles, seems to be working nicely!

[j9liu]

@ptrgags updated!

[ptrgags]

@j9liu the update function looks a lot nicer now!, I'll merge once CI passes

[lilleyse]

🥇 on the update cleanup.