PR Summary

OK, third time is the charm.

Previous overhaul packaged an inner and outer axes in a container "ColorbarAxes" and tried to dispatch methods between them.

This overhaul takes the much simpler approach of resizing the image using a custom _axes_locator that a) calls any existing locator b) or just uses the axes default position. The custom _axes_locator then shrinks the axes in the appropriate direction to make room for extend tri/rectangles. As with the previous fix, the extend tri/rectangles are drawn as patches in axes co-ordinates, rather than pcolormesh in "data" co-ordinates.

This addresses the concerns and closes #20479

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).