Generate face normals in pixel shader

[david-lively]

The glTF 2.0 specification requires that flat normals are generated when they are missing from the source data.

This PR modifies the CesiumDefaultTilesetShader to optionally generate face normals in the pixel shader. This is more space-efficient than the previous approach of duplicating vertices. The new boolean shader uniform, computeFlatNormals controls this behavior.

This feature will be enabled when all of these conditions are true:

1. The model does not have vertex normals
2. Generate Smooth Normals is not enabled
3. The material is lit, or the material is unlit and Cesium3DTileset.ignoreKhrMaterialsUnlit is true.

int32_t computeFlatNormalsPropertyID =   materialProperties.getComputeFlatNormalsID();
bool computeFlatNormals = !primitiveInfo.hasNormals &&
    !primitiveInfo.isUnlit &&
     primitive.mode != MeshPrimitive::Mode::POINTS;
material.SetFloat(computeFlatNormalsPropertyID, computeFlatNormals);

Note that any custom material that previously relied on CPU-generated flat normals should be updated to use the new feature. In CesiumDefaultTilesetShader, the necessary Shader Graph block is imlemented like so:

[j9liu]

Thanks @david-lively! I still have to test this on my own end, but I took a look through the code so I could get feedback to you sooner than later. No major changes, just some cleanup + word tweak suggestions ^^

[kring]

@david-lively I'm going to move this out of the September release because I think you're still working on it, and it's not critical to the release.