[tx] [ufowrite] Fix postscriptUnderline rounding

[josh-hadley]

Description

Update ufowrite (tx) to use floor() instead of round() when writing out postscriptUnderlineThickness and postscriptUnderlinePosition to fix a recently-noted bug which was increasing the value by 1 upon writing. Also added a test to check, as there was not anything in place that tickled this bug.

Fixes #1534

Checklist:

• I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[kaydeearts]

LGTM

[skef]

LGTM