glTF exporter: Deduplicate cameras, animation sampler and accessors

io_scene_gltf2/blender/exp/gltf2_blender_gather_accessors.py

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

io_scene_gltf2/blender/exp/gltf2_blender_gather_cameras.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_nodes.py

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

io_scene_gltf2/blender/exp/gltf2_blender_gather_skins.py

io_scene_gltf2/io/exp/gltf2_io_binary_data.py