SVG: improve handling of visibility property

visible children of an invisible parent are now visible

Change-Id: I2eafbd15d22f54ec39f12bfd32175925ab8a6184
Reviewed-on: https://gerrit.libreoffice.org/12504
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>