glTF: Better uri management

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py

io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py

io_scene_gltf2/blender/imp/gltf2_blender_image.py

io_scene_gltf2/io/com/gltf2_io_path.py

io_scene_gltf2/io/imp/gltf2_io_gltf.py