glTF exporter: fix shapekeys animation export

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/exp/gltf2_blender_extract.py

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

io_scene_gltf2/blender/exp/gltf2_blender_gather_mesh.py

...cene_gltf2/blender/exp/gltf2_blender_gather_primitives.py