xfig: optimize draw_(ellipse|polygon|rect) to combine stroke and fill

Tested with a modified render-test.dia because the original converted to
.fig make xfig 3.2.5c crash on every linestyle other than solid or dotted.

Also tested via xfig plug-in round-trip. For some reason the new single
objects for ellipse, rect and polygon are still put into a group. But
that's an issue to be tackled in xfig-import.c if at all.