tight_layout.get_renderer() does the right dance to set up a
renderer for any canvas. Perhaps it also should be moved somewhere
more easily findable and it should be better disambiguated with
backend_bases._get_renderer(), but that can be left for another
time...

Accidentally broke this in #16171.

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).