Ensure segmented polys have outlines

In the event the calculation removes outlines, we need to keep the
original polygon

Fixes https://gitlab.com/kicad/code/kicad/issues/5594

libs/kimath/src/geometry/shape_poly_set.cpp

@@ -1965,6 +1965,9 @@ static void partitionPolyIntoRegularCellGrid(
     {
         aOut.AddOutline( ps2.COutline( i ) );
     }
+
+    if( !aOut.OutlineCount() )
+        aOut = aPoly;
 }