glTF exporter: define a user extension API

io_scene_gltf2/__init__.py

...nder/exp/gltf2_blender_gather_animation_channel_target.py

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

...f2/blender/exp/gltf2_blender_gather_animation_samplers.py

...cene_gltf2/blender/exp/gltf2_blender_gather_animations.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_cameras.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py

...ltf2_blender_gather_material_normal_texture_info_class.py

...2_blender_gather_material_occlusion_texture_info_class.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py

.../gltf2_blender_gather_materials_pbr_metallic_roughness.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_mesh.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_nodes.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_sampler.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_skins.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_texture.py

...ne_gltf2/blender/exp/gltf2_blender_gather_texture_info.py

io_scene_gltf2/io/exp/gltf2_io_user_extensions.py