use projection before calling is_path_simple in rouinded_prism

rewrite vnf_bend so it doesn't rely on pseudo 3d polygon self-intersection

rounding.scad

@@ -1931,8 +1931,8 @@ function rounded_prism(bottom, top, joint_bot=0, joint_top=0, joint_sides=0, k_b
                                  top_patch[i][4][4]
                              ]
              ],
-     top_simple = is_path_simple(faces[0],closed=true),
-     bot_simple = is_path_simple(faces[1],closed=true),
+     top_simple = is_path_simple(project_plane(faces[0],faces[0]),closed=true),
+     bot_simple = is_path_simple(project_plane(faces[1],faces[1]),closed=true),
      // verify vertical edges
      verify_vert =
        [for(i=[0:N-1],j=[0:4])