crashtesting: tdf111681-6.pptx

regression from
    commit e5012e53
    use scanline when reading pixel data

Change-Id: Ic9382426191d5cbbffc6c3fd6f7038ed93715b8e
Reviewed-on: https://gerrit.libreoffice.org/49598Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>

vcl/source/gdi/bitmapex.cxx

@@ -1315,7 +1315,7 @@ void BitmapEx::setAlphaFrom( sal_uInt8 cIndexFrom, sal_Int8 nAlphaTo )
             Scanline pScanlineRead = pReadAccess->GetScanline( nY );
             for ( long nX = 0; nX < pReadAccess->Width(); nX++ )
             {
-                const sal_uInt8 cIndex = pReadAccess->GetIndexFromData( pScanlineRead, nX );
+                const sal_uInt8 cIndex = pReadAccess->GetPixelFromData( pScanlineRead, nX ).GetBlueOrIndex();
                 if ( cIndex == cIndexFrom )
                     pWriteAccess->SetPixelOnData( pScanline, nX, BitmapColor(nAlphaTo) );
             }