EMF+: force canvas if EMF+ comments are used.

Without this the drawinglayer renderer is used, whereas
only the canvas renderer supports EMF+.

Change-Id: Id8a10e400e08c1913e7d6864a51f7b73fc9be437
(cherry picked from commit be7d87bea2611eb316b9b379aebc031179d4f794)
Reviewed-on: https://gerrit.libreoffice.org/6940
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>