glTF: Removing one model breaks an other model

Reproduce:
- Insert two models
- Play first model
- Delete second model
Result: crash (windows) or first model disappear from the
opengl window (linux).

Change-Id: I940960754f42a2e4faf5b9f37c0d4a3c155e4650
(cherry picked from commit 30fcee6ce56ec4e50ee86c308ca5621746d817d5)
Reviewed-on: https://gerrit.libreoffice.org/10219
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>