improve import of masking-path-08-b.svg of the W3C svg test suite when using insert->media->image

Change-Id: Ib4d48165f982e394e2171ac82e6cc06911553904
Reviewed-on: https://gerrit.libreoffice.org/12168Reviewed-by: Christina Roßmanith <ChrRossmanith@web.de>
Tested-by: Christina Roßmanith <ChrRossmanith@web.de>