`cmptxtrender` identifies differences when comparing a font file with itself

[bobh0303]

In the following "regression" style test, cmptxtrender is asked to compare rendering of a font file with that of the same font file, but it reports differences:

$ cd font-scheherazade/references/4.000
$ cmptxtrender -e ot -e ot -s "arab" -l "" -t "../../tests/AllChars-auto.ftml" --outputtype json -g ../../source/glyph_data.csv "ScheherazadeNew-Regular.ttf" "ScheherazadeNew-Regular.ttf"
{
"U+060C" : [[["comma-ar.downward", 0.0, 0.0], ["_adv_", 929.0, 0.0]], [["comma-ar", 0.0, 0.0], ["_adv_", 929.0, 0.0]]],
"U+061B" : [[["semicolon-ar.downward", 0.0, 0.0], ["_adv_", 929.0, 0.0]], [["semicolon-ar", 0.0, 0.0], ["_adv_", 929.0, 0.0]]],
"":[]}
$

NB: This failure was seen in Sch project as a result of smith test which was comparing rendering of results/*.ttf with corresponding fonts in references/, so passing the same font file as the two arguments to cmptxtrender is not the issue, but is done here to make it easier to replicate and possibly debug.

[bobh0303]

Observations:

• all the false regression failures I see in smith test are tests involving font features (in the above it was cv84=1) but not all tests involving features were failures.
• Trying to replicate this with a plain-text input file and -f cv84=1 on the command-line has, so far, not been successful.

[bobh0303]

A short ftml file that demonstrates the problem .... just remove the .txt extension first. short.ftml.txt