[BUG] vnf_split() gets "Found non-coplanar face." assert fails.

[revarbat]

Code To Reproduce Bug

     path = [
        each arc(r=8, corner=[[0,-20],[20,-20],[15,20]]),
        each arc(r=8, corner=[[20,-20],[15,20],[0,20]]),
     ];
     tex = texture("vnf_dots");
     textured_revolution(path, closed=false, texture="vnf_dots", tex_size=[5,5], samples=4);

Expected behavior Should create a rounded cylindrical shape with bump textures.

What we get instead

ERROR: Assertion 'plane' failed: "Found non-coplanar face." in file vnf.scad, line 837
TRACE: called by 'flatten' in file vnf.scad, line 833
TRACE: called by '_slice_3dpolygons' in file vnf.scad, line 771
TRACE: called by 'vnf_slice' in file skin.scad, line 3019
TRACE: called by 'textured_revolution' in file skin.scad, line 3220
TRACE: called by 'textured_revolution' in file foo.scad, line 12