Decide whether usage of KHR_material_variants should be mandatory about openmaterial HOT 1 CLOSED

From multiple discussions, it became clear that parallel implementation of PBR materials and OpenMATERIAL materials within the glTF materials array is the way to go. Usage of the KHR_material_variants extension should be preserved for use cases where a "material switching" is explicitly needed. For illustration of the parallel implementation, an example was created: https://github.com/LudwigFriedmann/OpenMATERIAL/blob/master/objects/cube_pbr_gold.gltf

from openmaterial.