glTF: Replace Separate RGB shader node by new Separate Color node

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py

...ltf2/blender/imp/gltf2_blender_KHR_materials_clearcoat.py

...e_gltf2/blender/imp/gltf2_blender_pbrMetallicRoughness.py