Add generated classes for KHR_materials_variants

[javagl]

Add the information for the KHR_materials_variants extension to the tools/generate-classes/glTF.json configuration file, and re-generate the glTF classes.

The extension is located in the main glTF object, and in mesh.primitive objects. We can argue about the naming:

• The first one is called ExtensionKhrMaterialsVariants
• The second one is called ExtensionKhrMaterialsVariantsMeshPrimitive (quite a mouthful...)

The resulting structures can be used as follows:

^{Updated based on review comments}

void printMaterialVariantsInfo(const CesiumGltf::Model &model) {

  SPDLOG_INFO("Material variants info");

  bool isPresentOnGltf =
      model.hasExtension<CesiumGltf::ExtensionModelKhrMaterialsVariants>();
  SPDLOG_INFO("  Extension present on glTF? {}", isPresentOnGltf);
  if (!isPresentOnGltf) {
    return;
  }
  const CesiumGltf::ExtensionModelKhrMaterialsVariants *materialVariantsGltf =
      model.getExtension<CesiumGltf::ExtensionModelKhrMaterialsVariants>();
  const std::vector<CesiumGltf::ExtensionModelKhrMaterialsVariantsValue>
      variants = materialVariantsGltf->variants;
  for (const CesiumGltf::ExtensionModelKhrMaterialsVariantsValue &variant :
       variants) {
    SPDLOG_INFO("    Variant name: {}", variant.name);
  }

  const std::vector<CesiumGltf::Mesh> meshes = model.meshes;
  for (size_t m = 0; m < meshes.size(); m++) {
    const CesiumGltf::Mesh &mesh = meshes[m];
    const std::vector<CesiumGltf::MeshPrimitive> meshPrimitives =
        mesh.primitives;
    for (size_t p = 0; p < meshPrimitives.size(); p++) {
      const CesiumGltf::MeshPrimitive &meshPrimitive = meshPrimitives[p];
      bool isPresentOnMeshPrimitive = meshPrimitive.hasExtension<
          CesiumGltf::ExtensionMeshPrimitiveKhrMaterialsVariants>();
      SPDLOG_INFO("  Extension present on mesh primitive {} of mesh {}? {}", p,
                  m, isPresentOnMeshPrimitive);
      if (!isPresentOnMeshPrimitive) {
        continue;
      }
      const CesiumGltf::ExtensionMeshPrimitiveKhrMaterialsVariants
          *materialVariants = meshPrimitive.getExtension<
              CesiumGltf::ExtensionMeshPrimitiveKhrMaterialsVariants>();
      const std::vector<
          CesiumGltf::ExtensionMeshPrimitiveKhrMaterialsVariantsMappingsValue>
          mappings = materialVariants->mappings;
      for (const CesiumGltf::
               ExtensionMeshPrimitiveKhrMaterialsVariantsMappingsValue
                   &mapping : mappings) {
        SPDLOG_INFO("    Mapping:");

        SPDLOG_INFO("      Variant:");
        const std::vector<int32_t> mappingVariants = mapping.variants;
        for (const int32_t mappingVariant : mappingVariants) {
          SPDLOG_INFO("        {}", mappingVariant);
        }
        SPDLOG_INFO("      Material: {}", mapping.material);
        SPDLOG_INFO("      Name: {}", mapping.name.value_or("(none)"));
      }
    }
  }
}

(This code is taken from https://github.com/javagl/cesium-cpp/tree/materials-variants-test )

[lilleyse]

If an extension has multiple attachment points like KHR_materials_variants does, the naming convention is Extension${AttachmentName}${ExtensionName} (see EXT_structural_metadata for example):

• ExtensionKhrMaterialsVariants should be ExtensionModelKhrMaterialsVariants
• ExtensionKhrMaterialsVariantsMeshPrimitive should be ExtensionMeshPrimitiveKhrMaterialsVariants.

[javagl]

I should have noticed that. The code (and the example code) have been updated.

[kring]

This looks good to me, thanks @javagl! @lilleyse since you did some reviewing, I'll let you hit merge when you're happy.