glTF exporter: Manage skinning when some vertices are not weights at all

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/exp/gltf2_blender_extract.py

io_scene_gltf2/blender/exp/gltf2_blender_gather.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_skins.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_tree.py

io_scene_gltf2/io/imp/gltf2_io_binary.py