Implement constant LOD glTF extension (WIP)

[danielzhong]

Description

See WIP extension spec: CesiumGS/glTF#92

Simple test model that uses EXT_textureInfo_constant_lod: constant_lod_example.zip

const viewer = new Cesium.Viewer("cesiumContainer", {
  infoBox: false,
  selectionIndicator: false,
  shadows: true,
  shouldAnimate: true,
});

function createModel(url, height) {
  viewer.entities.removeAll();

  const position = Cesium.Cartesian3.fromDegrees(
    -123.0744619,
    44.0503706,
    height
  );

  const heading = Cesium.Math.toRadians(0.0);
  const pitch = 0.0;
  const roll = 0.0;
  const hpr = new Cesium.HeadingPitchRoll(heading, pitch, roll);
  const orientation = Cesium.Transforms.headingPitchRollQuaternion(position, hpr);

  const entity = viewer.entities.add({
    name: url,
    position: position,
    orientation: orientation,
    model: {
      uri: url,
      minimumPixelSize: 128,
      maximumScale: 20000,
    },
  });

  viewer.trackedEntity = entity;
}

createModel("http://172.20.20.20:8081/example.gltf", 100.0);

Issue number and link

#12892

Testing plan

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

[github-actions]

Thank you for the pull request, @danielzhong!

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