glTF exporter: Option to not export texture images

io_scene_gltf2/__init__.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_texture.py

io_scene_gltf2/blender/exp/gltf2_blender_get.py