glTF importer: Manage KHR_texture_transform extension

io_scene_gltf2/__init__.py

.../imp/gltf2_blender_KHR_materials_pbrSpecularGlossiness.py

io_scene_gltf2/blender/imp/gltf2_blender_image.py

io_scene_gltf2/blender/imp/gltf2_blender_map_emissive.py

io_scene_gltf2/blender/imp/gltf2_blender_map_normal.py

io_scene_gltf2/blender/imp/gltf2_blender_map_occlusion.py

...e_gltf2/blender/imp/gltf2_blender_pbrMetallicRoughness.py

io_scene_gltf2/blender/imp/gltf2_blender_texture.py

io_scene_gltf2/io/imp/gltf2_io_gltf.py