FIX: Prevent set_alpha from changing color of legend patch

[StefRe]

of polycollection (e.g. from fill_between) by updating original edge and face color too.

Closes #21966

PR Summary

When updating the properties of the legend patch from the PolyCollection returned by fill_between, the HandlerPolyCollection failed to set the original edge and face colors so that when later setting the alpha of the patch, its color was set to the default C0.
This was fixed by updating the original edge and face colors too.

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [n/a] New features are documented, with examples if plot related.
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).

[tacaswell]

@StefRe Do you think there something reasonable thing to search on to see if we have any other moles of this category that need to be whacked?

[StefRe]

@tacaswell
I tested all plot types and the custom legend types from the legend guide and they're all OK. So I'm pretty confident that there no more instances of this issue, but in the end this is just a subjective assessment, not a automatic code review.

[tacaswell]

It is much easier to prove a bug exists than it does not exist ;) I was thinking of a heuristiic like "skimmed all of the functions called def _update_prop that exist in the code base and they all remember to copy the original colors", but testing that it works is better 👍🏻 . Thank you for the diligence.

[StefRe]

@tacaswell a very rough heuristics would be looking around ...facecolor assignments and see if other ...facecolor properties (if they exist) are assigned too:

grep -r --include=*.py --exclude=test*.py -C5 -n 'facecolor = [a-zA-Z_][a-zA-Z0-9_\.]*facecolor' *.py | grep 'facecolor\|^\-'

which yields

collections.py:918:        self._original_facecolor = other._original_facecolor
collections.py-919-        self._facecolors = other._facecolors
--
legend_handler.py-789-        legend_handle._facecolor = first_color(orig_handle.get_facecolor())
legend_handler.py:791:        legend_handle._original_facecolor = orig_handle._original_facecolor
--
lines.py:1269:        self._markerfacecolor = other._markerfacecolor
lines.py-1270-        self._markerfacecoloralt = other._markerfacecoloralt
--
patches.py:257:        self._facecolor = other._facecolor
patches.py:259:        self._original_facecolor = other._original_facecolor

According to this, everything seems to be fine :).