Update libgltf: various fixes, memory usage optimization (second try)

Fixes in the new tarball (beside the upstreamed bug fixes):
- In walkthrough mode camera is moved relative to the current
camera position and not relative to the initial camera position.
- The right FPS numbers are displayed under Linux.
- Some memory optimization to avoid running out of memory
in case of huge models.
- Other freedesktop bugs: fdo#81205, fdo#81207, fdo#81250

Change-Id: Ie0f65be3312688196fcca634eb396c577f991073
Reviewed-on: https://gerrit.libreoffice.org/10274
Reviewed-by: Zolnai Tamás <zolnaitamas2000@gmail.com>
Tested-by: Zolnai Tamás <zolnaitamas2000@gmail.com>