glTF: crash after starting a model without an OpenGL window

Change-Id: I726f4d62ab062fbd41376183f60c26d8e6cfe33e
(cherry picked from commit 6174152cfffd8e31b9eda238990a161c8c2b9a71)
Reviewed-on: https://gerrit.libreoffice.org/10218
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>