Adds dynamic model/node transformations to ModelExperimentalNode

[sanjeetsuhag]

This PR adds support for dynamically changing node transforms in ModelExperimental. It also adds the ability to traverse the scene graph of ModelExperimentaNode objects.

Architecture

model.modelMatrix update

node.transform update

To-Do

• Add a way to update the draw commands with the new modelMatrix
• Fix child transform bug
• Update documentation
• Add tests

Testing

Truck Sandcastle

[cesium-concierge]

Thanks for the pull request @sanjeetsuhag!

• ✔️ Signed CLA found.
• ❔ 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.

[sanjeetsuhag]

Will users expect the exact same indices as in the glTF? Because the runtimeNodes array is generated from a post-order depth-first traversal of the scene graph, a node's glTF ID will not necessarily be a match to its index in the runtimeNodes array.

[sanjeetsuhag]

@ptrgags This is ready for review.

[ptrgags]

@sanjeetsuhag just a few minor comments

[ptrgags]

Looks good to me, @lilleyse do you want to take a look before I merge?

[lilleyse]

@ptrgags feel free to merge

[ptrgags]

Thanks @sanjeetsuhag!