It is possible ImpGraphic::ImplSetPrepared() should handle this
directly, though.

Change-Id: I1a020a45eaded140493d7207f8e0e0dfe551cc88
Reviewed-on: https://gerrit.libreoffice.org/57623
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>