Merge topic 'gltf_scene'

48eac350 GLTF document loader caches a default scene

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Reviewed-by: Alexy Pellegrini <alexy.pellegrini@kitware.com>
Reviewed-by: Dan Lipsa <dan.lipsa@kitware.com>
Merge-request: !11836