tdf#108973 writerfilter: allow textboxes to be in the background

Word appears to use negative z-indexes to indicate behind-text
zordering and positive numbers for in-front placement. This
was added for shapes in LO4.3, now applying to textboxes also.

Change-Id: I3b06fb231329f151ca978f3a68b4d4e89bc28515
Reviewed-on: https://gerrit.libreoffice.org/39671Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Justin Luth <justin_luth@sil.org>

sw/qa/extras/ooxmlexport/ooxmlexport7.cxx

@@ -641,6 +641,16 @@ DECLARE_OOXMLEXPORT_TEST(testTdf77219_foregroundShape, "tdf77219_foregroundShape
     CPPUNIT_ASSERT_EQUAL_MESSAGE("Shape is in front of the paragraph", true, bool(getProperty<bool>(getShape(1), "Opaque")));
 }
 
+DECLARE_OOXMLEXPORT_TEST(testTdf108973_backgroundTextbox, "tdf108973_backgroundTextbox.docx")
+{
+    CPPUNIT_ASSERT_EQUAL_MESSAGE("Textbox is in front of the paragraph", false, bool(getProperty<bool>(getShape(1), "Opaque")));
+}
+
+DECLARE_OOXMLEXPORT_TEST(testTdf108973_foregroundTextbox, "tdf108973_foregroundTextbox.docx")
+{
+    CPPUNIT_ASSERT_EQUAL_MESSAGE("Textbox is in front of the paragraph", true, bool(getProperty<bool>(getShape(1), "Opaque")));
+}
+
 DECLARE_OOXMLEXPORT_TEST(testPresetShape, "preset-shape.docx")
 {
     // Document contains a flowChartMultidocument preset shape, our date for that shape wasn't correct.

writerfilter/source/dmapper/DomainMapper_Impl.cxx

@@ -2039,6 +2039,7 @@ void DomainMapper_Impl::PushShapeContext( const uno::Reference< drawing::XShape
                         aGrabBag[i].Value >>= zOrder;
                         xShapePropertySet->setPropertyValue( "ZOrder", uno::makeAny(pZOrderHelper->findZOrder(zOrder)));
                         pZOrderHelper->addItem(xShapePropertySet, zOrder);
+                        xShapePropertySet->setPropertyValue(getPropertyName( PROP_OPAQUE ), uno::makeAny( zOrder >= 0 ) );
                         checkZOrderStatus = true;
                     }
                     if(checkBtLrStatus && checkZOrderStatus)