Add support for point clouds in Unity

[j9liu]

This PR builds off of CesiumGS/cesium-native#591 and adds support for rendering point clouds (pnts content) in Unity.

Some screenshots:

Melbourne Point Cloud

Mount St. Helen	Church

CesiumJS sandcastle with the above models for reference

[lilleyse]

@j9liu this is exciting to see!

The colors look a bit faded and I wonder if you need to do an sRGB -> linear conversion in cesium-native. pnts is assumed to be sRGB whereas glTF vertex colors are assumed to be linear. I'm not sure what Unity expects as its input.

Peter ran into a similar issue in CesiumJS: CesiumGS/cesium#9978 (comment)

[j9liu]

@lilleyse Thanks for the catch! It does look more accurate with the colors converted to linear space:

Will push my changes to cesium-native once I update the unit tests as well.

[kring]

Looks great!