tdf#156508 Blur shadow is solid in edit and presentation mode

regression from
    commit 81994cb2b8b32453a92bcb011830fcb884f22ff3
    Author: Noel Grandin <noelgrandin@gmail.com>
    Date:   Fri Apr 16 20:33:10 2021 +0200
    Convert internal vcl bitmap formats transparency->alpha (II)

Change-Id: I14f62547e3301cab0eff79716bf7bc2a51ead563
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/155338
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
diff --git a/drawinglayer/source/primitive2d/GlowSoftEgdeShadowTools.cxx b/drawinglayer/source/primitive2d/GlowSoftEgdeShadowTools.cxx
index 9bbdf71..3c45fdd 100644
--- a/drawinglayer/source/primitive2d/GlowSoftEgdeShadowTools.cxx
+++ b/drawinglayer/source/primitive2d/GlowSoftEgdeShadowTools.cxx
@@ -58,10 +58,10 @@ AlphaMask ProcessAndBlurAlphaMask(const Bitmap& rMask, double fErodeDilateRadius
    else if (fErodeDilateRadius < 0)
        BitmapFilter::Filter(mask, BitmapErodeFilter(-fErodeDilateRadius, 0xFF));

    if (nTransparency != 255)
    if (nTransparency)
    {
        const Color aTransparency(nTransparency, nTransparency, nTransparency);
        mask.Replace(COL_WHITE, aTransparency);
        mask.Replace(COL_BLACK, aTransparency);
    }

    // We need 8-bit grey mask for blurring
@@ -72,8 +72,6 @@ AlphaMask ProcessAndBlurAlphaMask(const Bitmap& rMask, double fErodeDilateRadius

    mask.Scale(rMask.GetSizePixel());

    mask.Invert(); // convert transparency to alpha

    return AlphaMask(mask.GetBitmap());
}