Merge topic 'fix_obj_importer_mtllib'

bc2c4c30 OBJImporter: Use ImageReaderFactory for textures
f06914d6 OBJImporter: Add a test for default mtllib and texturepath
3749bc57 OBJImporter: Improve testing logic
45b35403 OBJImporter: Document default behaviors
2034b880 OBJImporter: Better default TexturePath
3237edc4 OBJImporter: Read mtllib keyword when parsing OBJ

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Reviewed-by: Michael Migliore <mcmigliore@gmail.com>
Merge-request: !11922