fdo#80020 Fix regression for style sheet inheritance

The inheritance relationship of styles in Draw was lost when writing files.

Change-Id: I81588a031bf4c649338fad9b7342fbbe11df9aef
Reviewed-on: https://gerrit.libreoffice.org/10309
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>