Greatly improve performance of clamped Entity geometry with dynamic colors

[mramato]

When we originally added support for GroundPrimitive, it did not support per-instance colors. We fixed this some time ago but apparently the Entity API or ClassificationPrimitive was never properly updated to take advantage of it.

The below Sandcastle code is unusably slow in master, in this branch is flies.

const viewer = new Cesium.Viewer('cesiumContainer', {
    terrainProvider: Cesium.createWorldTerrain()
});

const color = new Cesium.CallbackProperty(() => {
    return Cesium.Color.fromRandom({
        alpha: 1
    });
}, false);

const granularity = 3;
for (let lon = -180.0; lon < 180.0; lon += granularity) {
    for (let lat = -90.0; lat < 90.0; lat += granularity) {
        const entity = viewer.entities.add(new Cesium.Entity({
            rectangle: {
                coordinates: Cesium.Rectangle.fromDegrees(lon, lat, lon + granularity, lat + granularity),
                material: new Cesium.ColorMaterialProperty(color)
            }
        }));
    }
}

console.log(`Created ${viewer.entities.values.length} entities`);

I have some failing tests that need to be fixed, but I wanted to get the PR open to ensure that there wasn't some other reason this limitation was in place that I missed. Especially my changes to the ClassificationPrimitive and Primitive. CC @likangning93 @lilleyse

[cesium-concierge]

Thanks for the pull request @mramato!

• ✔️ 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.
• Works (or fails gracefully) in IE11.

[mramato]

Assuming no surprises, it would be nice to get this into the next reelase.

[hpinkos]

Looks great to me @mramato!

[mramato]

Turns out the tests were an easy fix, so CI should be happy now. @lilleyse what are your thoughts here?

[lilleyse]

👍 from me