glTF exporter: manage delta transform animation

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/com/gltf2_blender_data_path.py

...f2/blender/exp/gltf2_blender_gather_animation_channels.py

...r/exp/gltf2_blender_gather_animation_sampler_keyframes.py