glTF: Merge glTF Material Output nodes into a single one

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/com/gltf2_blender_material_helpers.py

io_scene_gltf2/blender/com/gltf2_blender_ui.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py

...f2/blender/exp/gltf2_blender_gather_materials_specular.py

io_scene_gltf2/blender/exp/gltf2_blender_get.py

...ne_gltf2/blender/imp/gltf2_blender_KHR_materials_sheen.py

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