Watermark was drawn under the page background.
It has to be placed on the upper layer to be visible.

Change-Id: I132a313eed6fb712aafdca14a38fe559aa4231c8
Reviewed-on: https://gerrit.libreoffice.org/41289Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Szymon Kłos <szymon.klos@collabora.com>