Conservative clipping for parallel case

When clipping edge and polygon edge is parallel and overlapping,
`is_inside()` and `edge_intersection()` gives non-consistant results. In
such cases, conservatively preserve the polygon vertex for clipping.

src/ezdxf/math/clipping.py

@@ -154,10 +154,10 @@ def edge_intersection() -> Vec2:
                 # next polygon edge to test: edge_start -> edge_end
                 if is_inside(edge_end):
                     if not is_inside(edge_start):
-                        clipped.append(edge_intersection())
+                        clipped.append(edge_intersection() or edge_start)
                     clipped.append(edge_end)
                 elif is_inside(edge_start):
-                    clipped.append(edge_intersection())
+                    clipped.append(edge_intersection() or edge_end)
                 edge_start = edge_end
             clip_start = clip_end
         return (clipped,)