Handle unlit materials

[lilleyse]

If a glTF uses the KHR_materials_unlit extension we should render with an unlit material and not generate normals.

We could also give some control to the user. There could be a drop-down list to override the default behavior. The options would be UNLIT, LIT, or AUTO.

[lilleyse]

Cesium for Unreal supports unlit materials by disabling shadows and making all normals point up (along the ellipsoid surface normal: CesiumGS/cesium-unreal#1017

I tried experimenting with the same idea in Omniverse and the results don't look great. Aside from ray self-intersections which show up as black, there's this flat shading type of look regardless of the normal direction. It reminds me of some of the issues in #59.

Smooth normals	Flat normals	Up normals

unlit-normals-test.usda.zip

Here's the code:

pxr::VtArray<pxr::GfVec3f> getNormalsGlobe(
    const pxr::VtArray<pxr::GfVec3f>& positions,
    const glm::dmat4& localToEcefTransform) {

    const auto ecefToLocalTransform = glm::dmat3(glm::inverse(localToEcefTransform));

    pxr::VtArray<pxr::GfVec3f> normalsUsd(positions.size(), pxr::GfVec3f(0.0f));

    for (size_t i = 0; i < positions.size(); i++) {
        const auto position = UsdUtil::usdToGlmVector(positions[i]);
        const auto positionEcef = glm::dvec3(localToEcefTransform * glm::dvec4(position, 1.0));
        const auto normalEcef = glm::normalize(positionEcef); // geocentric normal, should be geodetic
        const auto normal = ecefToLocalTransform * normalEcef;
        normalsUsd[i] = pxr::GfVec3f(static_cast<float>(normal.x), static_cast<float>(normal.y), static_cast<float>(normal.z));
    }

    return normalsUsd;
}