poke libgltf towards working with a core profile as available under gtk3

Change-Id: I48a5a33c6d401e0aa3e6d18f93a498465eda8634
Reviewed-on: https://gerrit.libreoffice.org/41999Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>