glTF: More rendering related error handling

If any error occurs related to glTF rendering, a question
mark is displayed at the place of the OpenGL window
(Question mark is a general concept of media objects).

Change-Id: Id7df5a47a4dd6832e640b3a7cb3c7712ebfb4408
Update libgltf: various fixes, memory usage optimization

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: Iba90d43e13dac7792072877d83d17fb1bcbfaae4