tdf#139759 sw ms-formats: export comment char background

EditEngine's character background is a SvxColorItem,
while Writer uses a SvxBrushItem. Thus we have no automatic
transition possible between the two, and during export it
was just being ignored as > RES_UNKNOWNATR_BEGIN.

A unit test will have to wait until import (of shd) works.

P.S. Import of "highlight" was fixed in an earlier commit.
Setting Options - Save - MS Office Compat - As Highlighting
would fully work now.

Change-Id: I8674173a3d1bf1581c09458c3087d6fd39e48a1c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/137090
Tested-by: Justin Luth <jluth@mail.com>
Reviewed-by: Justin Luth <jluth@mail.com>
diff --git a/sw/source/filter/ww8/wrtw8esh.cxx b/sw/source/filter/ww8/wrtw8esh.cxx
index bcfc410..1d8c993 100644
--- a/sw/source/filter/ww8/wrtw8esh.cxx
+++ b/sw/source/filter/ww8/wrtw8esh.cxx
@@ -33,6 +33,7 @@
#include <svx/svdobj.hxx>
#include <svx/svdotext.hxx>
#include <svx/svdpage.hxx>
#include <editeng/colritem.hxx>
#include <editeng/outlobj.hxx>
#include <editeng/editobj.hxx>
#include <editeng/brushitem.hxx>
@@ -1157,6 +1158,12 @@ void MSWord_SdrAttrIter::OutAttr( sal_Int32 nSwPos )
                m_rExport.WriteChar(0x9);
                continue;
            }
            if (nWhich == EE_CHAR_BKGCOLOR)
            {
                Color aColor(static_cast<const SvxColorItem*>(rTextAtr.pAttr)->GetValue());
                m_rExport.AttrOutput().OutputItem(SvxBrushItem(aColor, RES_CHRATR_BACKGROUND));
                continue;
            }

            const sal_uInt16 nSlotId = pSrcPool->GetSlotId(nWhich);
            if (nSlotId && nWhich != nSlotId)