Heightfield() output fails vnf_validate()

[RAMilewski]

Running the 20x20 pixel png above through img2scad.py generates this file:

20x20S.txt (I changed the extension from .scad to .txt because GitHub refuses to load .scad files here.)

include <BOSL2/std.scad>
include <tex/20x20S.scad>

vnf =   heightfield(custom, size = [20,20], bottom = 0, maxz = 1);
vnf_validate(vnf);

...results in: ...and the following console messages: validate_messages.txt

It also appears that the rendering attempt is not symmetrical while the input png is.