sw XHTML export: avoid writing default transparent background for ReqIF

sw XHTML export: avoid writing default transparent background for ReqIF

We started writing properties of tables and rows since commit
c3c3303516c3da9372dce3f05f38f15a104e961c (sw XHTML export: output table
/ table row background format using CSS, 2022-05-10).

In case the SwTableLine has an explicit SvxBrushItem with its color set
to COL_TRANSPARENT, we turn that into a "background: transparent" CSS by
default. This is a 1:1 mapping from the doc model, but HTML defaults to
this already, so this is considered as noise.

Extend IgnorePropertyForReqIF() to filter out these unwanted defaults,
and fix SwHTMLWriter::OutCSS1_Property(), because it used to not pass
the CSS value for the filter function.

The behavior for table cells is unchanged, we continue to not export
cell properties (in the ReqIF case) at all.

Change-Id: Idbcd07809e159def694f4de017eebc7ad4104575
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/135576
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>