tdf#50613 add close_path to correctly show closed polygons

For closed polygons it is essential to add a close_path to
correctly show the last line join.

Change-Id: Ib6f37bbc5e85133f21a936b186eb0ab12773f7da

vcl/unx/generic/gdi/salgdi.cxx

@@ -829,6 +829,11 @@ bool X11SalGraphics::drawPolyLine(
                 }
             }
 
+            if(rPolygon.isClosed() && !bNoJoin)
+            {
+                cairo_close_path(cr);
+            }
+
             cairo_stroke(cr);
         }