fdo#79078 restore transparency in SVG export

Change-Id: I5bdf6c3b313a73126cb692e5101836f424a2fc08
Reviewed-on: https://gerrit.libreoffice.org/9442
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>