fdo#85478 Avoid destroying bitmaps after VCL is shut down

Change-Id: I1ece738e7f60b6bbbdc802339f8b9deec9396e1b
Reviewed-on: https://gerrit.libreoffice.org/12469
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>