glTF : fixes for emission export, correction nodes for lights & camera

io_scene_gltf2/__init__.py

...ene_gltf2/blender/exp/gltf2_blender_gather_light_spots.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_lights.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_nodes.py

io_scene_gltf2/blender/exp/gltf2_blender_get.py