public void registerMaterialAdapter(String gltfMaterialName, MaterialAdapter adapter) {
        materialAdapters.put(gltfMaterialName, adapter);
    }
Registers a MaterialAdapter for the given materialName.
The materialName must be "pbrMetallicRoughness" or any name from KHR_materials glTF Extension (for example "pbrSpecularGlossiness" for "KHR_materials_pbrSpecularGlossiness" extension)

@param gltfMaterialName the name of the gltf material
@param adapter          the material adapter
jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/GltfModelKey.java