tdf#141419 restore use any explicitly set background color in previews

regression since...

commit 6f3899b27156591e65f62649a92c727eb6f5dd03
Date:   Thu Oct 29 15:03:44 2020 +0100

    Resolves tdf#137059 - Use application colors for font preview

but continue to use the doc-color setting when there isn't an explicit
bg setting in the input propertyset

Change-Id: I531b5ddea7700fcd4a731f32d4830b4d0b767d7b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/114418
Tested-by: Jenkins
Reviewed-by: Xisco Fauli <xiscofauli@libreoffice.org>
(cherry picked from commit 40868f520f9da24d3eaa5e0489da8657bc8383b4)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/114425
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
Tested-by: Michael Stahl <michael.stahl@allotropia.de>
diff --git a/svx/source/dialog/fntctrl.cxx b/svx/source/dialog/fntctrl.cxx
index 1b2eb42..26e44a2 100644
--- a/svx/source/dialog/fntctrl.cxx
+++ b/svx/source/dialog/fntctrl.cxx
@@ -1008,6 +1008,16 @@ void SvxFontPrevWindow::SetFromItemSet(const SfxItemSet &rSet, bool bPreviewBack
    rCJKFont.SetTransparent( bTransparent );
    rCTLFont.SetTransparent( bTransparent );

    if( !bPreviewBackgroundToCharacter )
    {
        if( GetWhich( rSet, SID_ATTR_BRUSH, nWhich ) )
        {
            const SvxBrushItem& rBrush = static_cast<const  SvxBrushItem&>( rSet.Get( nWhich ) );
            if( GPOS_NONE == rBrush.GetGraphicPos() )
                pImpl->mxBackColor = rBrush.GetColor();
        }
    }

    // Font
    SetPrevFont( rSet, SID_ATTR_CHAR_FONT, rFont );
    SetPrevFont( rSet, SID_ATTR_CHAR_CJK_FONT, rCJKFont );