tdf#113254 fix regression - images not displayed in MSO

regression of commit	31ee13b5
    convert SHAPEFLAG defines to scoped enum ShapeFlag

This patch restores original behaviour
and re-adds clarified comment.

Change-Id: Ie4f75902611127ccef7fa1ca3d32b87f06675db4
Reviewed-on: https://gerrit.libreoffice.org/44424Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jochen Nitschke <j.nitschke+logerrit@ok.de>

filter/source/msfilter/escherex.cxx

@@ -5294,7 +5294,7 @@ void EscherEx::AddShape( sal_uInt32 nShpInstance, ShapeFlag nFlags, sal_uInt32 n
     if ( !nShapeID )
         nShapeID = GenerateShapeId();
 
-    if (!(nFlags & ShapeFlag::Group))
+    if (nFlags ^ ShapeFlag::Group) // no pure group shape
     {
         if ( mnGroupLevel > 1 )
             nFlags |= ShapeFlag::Child; // this not a topmost shape