Define \mathdefault as a noop in the usetex preamble.

[anntzer]

This avoids having to strip out \mathdefault (a non-existent latex
command) before generating usetex strings, as that's brittle on tickers
-- the global usetex flag may change after the ticker is instantiated.

Closes #10317.

(Note that we claim that the semantics of \mathdefault is "match the non-math font", but in usetex mode we previously just stripped it out anyways; making TeX actually use the non-math font in math would be... trickier.)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[timhoffm]

This should get documentation:

• User facing stating the de-facto way of working (https://matplotlib.org/devdocs/tutorials/text/mathtext.html in the section describing \mathtext.
• Comments in make_tex* that explain the reason for the noop command definition.

[anntzer]

This is already documented in https://matplotlib.org/devdocs/tutorials/text/mathtext.html (look up \mathdefault there).

Added comments, and also refactored the preamble-getting in to avoid duplicating that between make_tex and make_tex_preview.

[jklymak]

This looks good, but can this also get a test for the breaking case so it doesn't break again?

[anntzer]

sure... done

[jklymak]

Feel free to re-milestone and back port if we think thats important....

[pyZerrenner]

Thanks for fixing this! This solution is much simpler than my idea :)

[anntzer]

You are welcome :)

[tacaswell]

@meeseeksdev backport to v3.2.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.2.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 508f21fb42d172201e5048b701fa13e16868bac7

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #15695: Define \\mathdefault as a noop in the usetex preamble.'

4. Push to a named branch :

git push YOURFORK v3.2.x:auto-backport-of-pr-15695-on-v3.2.x

5. Create a PR against branch v3.2.x, I would have named this PR:

"Backport PR #15695 on branch v3.2.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.