In Rectangle: no need to keep track and update ofself._x1 and
self._x2 all the time, just when we compute the patch_transform
(which doesn't need to be split out into its own function, or stored
into an instance variable, given that it's never accessed via the
private attribute name).

In RegularPolygon: likewise, we can just compute the transform in
get_patch_transform(), and nowhere else, so the other attributes can
just be plain attributes instead of properties that update the transform
all the time.

PR Summary

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings andpydocstyle<4 and runflake8 --docstring-convention=all).
• New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).