• In common_texification.py, when splitting math and non-math segments,
  match two dollar signs surrounding a math segment rather than single
  dollar signs; this lets us correctly handle e.g. $2 which we
  explicitly support in other backends as being plain text, not
  unbalanced math (the previous implementation would handle that as
  math).
• Instead of adding \displaystyle in front of each and every math block,
  use the TeX-provided \everymath mechanism to just register that
  once (this basically auto-prepends \displaystyle before every math
  block).
• Define \mathdefault as a noop (which is consistent with what
  texmanager does) instead of stripping it out, which would allow e.g.
  for the possibility that someone redefines \mathdefault in whatever
  way they want.

PR Summary

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).