Fix handling of "d" glyph in backend_ps.

[anntzer]

PR Summary

Closes #19979.
A rather "interesting" bug :p

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[WeatherGod]

Correct me if I am wrong, but wouldn't the same problem happen if someone has a "_d" in their figure?

[anntzer]

Actually the problem is if someone has a font with a glyph whose name is _d, which I guess may be technically possible though likely in violation of the glyph naming rules at https://github.com/adobe-type-tools/agl-specification.
Even if we choose to not abbreviate bind def we can still end up running into a wall with really bad glyph names (e.g. a glyph named "bind" or "def"...), which probably don't exist in practice though. So I'd rather just stick with this fix for now, and if this really happens, then we can think of a general glyph renaming facility to just map them all to uuids :/

[WeatherGod]

Fair enough.