tdf#133944 writer image shadow incorrect

regression from
    commit 1f59cbe36c9899f6fa9a869331c9be454abd4606
    Date:   Fri Apr 17 15:38:05 2020 +0200
    simplify some SwRect code - use the SwRect Add* variants

Change-Id: I435b1153f272b08a2bfe7d90145de1ee5a702b08
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/97720
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
diff --git a/sw/source/core/layout/paintfrm.cxx b/sw/source/core/layout/paintfrm.cxx
index 335bb00..b6d5a91 100644
--- a/sw/source/core/layout/paintfrm.cxx
+++ b/sw/source/core/layout/paintfrm.cxx
@@ -4243,8 +4243,8 @@ static void lcl_PaintShadow( const SwRect& rRect, SwRect& rOutRect,
                if ( bDrawFullShadowRectangle )
                {
                    // draw full shadow rectangle
                    aOut.AddTop( nHeight );
                    aOut.AddLeft( nWidth );
                    aOut.Top( rOutRect.Top() + nHeight );
                    aOut.Left( rOutRect.Left() + nWidth );
                    aRegion.push_back( aOut );
                }
                else
@@ -4253,18 +4253,18 @@ static void lcl_PaintShadow( const SwRect& rRect, SwRect& rOutRect,
                    {
                        aOut.Top( rOutRect.Bottom() - nHeight );
                        if( bLeft )
                            aOut.AddLeft( nWidth );
                            aOut.Left( rOutRect.Left() + nWidth );
                        aRegion.push_back( aOut );
                    }
                    if( bRight )
                    {
                        aOut.Left( rOutRect.Right() - nWidth );
                        if( bTop )
                            aOut.AddTop( nHeight );
                            aOut.Top( rOutRect.Top() + nHeight );
                        else
                            aOut.Top( rOutRect.Top() );
                        if( bBottom )
                            aOut.AddBottom( - nHeight );
                            aOut.Bottom( rOutRect.Bottom() - nHeight );
                        aRegion.push_back( aOut );
                    }
                }
@@ -4280,8 +4280,8 @@ static void lcl_PaintShadow( const SwRect& rRect, SwRect& rOutRect,
                if ( bDrawFullShadowRectangle )
                {
                    // draw full shadow rectangle
                    aOut.AddBottom( - nHeight );
                    aOut.AddRight( - nWidth );
                    aOut.Bottom( rOutRect.Bottom() - nHeight );
                    aOut.Right( rOutRect.Right() - nWidth );
                    aRegion.push_back( aOut );
                }
                else
@@ -4290,7 +4290,7 @@ static void lcl_PaintShadow( const SwRect& rRect, SwRect& rOutRect,
                    {
                        aOut.Bottom( rOutRect.Top() + nHeight );
                        if( bRight )
                            aOut.AddRight( - nWidth );
                            aOut.Right( rOutRect.Right() - nWidth );
                        aRegion.push_back( aOut );
                    }
                    if( bLeft )