groff warning: TE macro called with TW register undefined

[Hugh McMaster <hugh.mcmaster@outlook.com>]

Lintian has recently started emitting this groff warning for ftlint.1
in freetype-demos as part of its man pages check:

groff-message an.tmac:<standard input>:66: warning: tbl preprocessor
failed, or it or soelim was not run; table(s) likely not rendered (TE
macro called with TW register undefined)

It seems TW is not defined in the number register.

While I can define TW by adding `.nr TW 0` before .TS, I don't know if
that's the correct solution.

Defining TW certainly prevents groff emitting the warning, but there
is no visual difference when viewing the man page with or without TW
defined.

Online searching yields a few results but seemingly few clues. The
warning comes from an upstream change.

Any ideas on fixing this warning, or is it best to ignore it for now?