fdo#86298 Avoid crash blending upside down Bitmaps of differing size

Change-Id: I3ed0dec7c2f8d80ca104f8ba8d47894ace582c0a
Reviewed-on: https://gerrit.libreoffice.org/12961
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>