ENH: clip_path keyword for contour and contourf

[rcomer]

PR summary

Closes #2369. Thanks to #25247, we now only need some very minor plumbing to expose clip_path in contour and contourf.

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.patches as mpatches

x = y = np.arange(-3.0, 3.01, 0.025)
X, Y = np.meshgrid(x, y)
Z1 = np.exp(-X**2 - Y**2)
Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
Z = (Z1 - Z2) * 2

fig, ax = plt.subplots()
patch = mpatches.RegularPolygon((0, 0), 5, radius=2, transform=ax.transData)
ax.contourf(X, Y, Z, clip_path=patch)

plt.show()

I'm unsure whether this should have an image test. For now, I just went for the simplest test that fails against main. I didn't add an example because we already have examples for both contours and clipping (the above, which is also the what's new entry, was adapted from this one and this one).

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[QuLogic]

If you do want in image test, it might be possible to use a check_figures_equal test instead of a new image.

[QuLogic]

I think we prefer data to be last, though I'm not sure about new arguments.

[rcomer]

https://github.com/matplotlib/matplotlib/compare/ea59f24a6b10c1a34cc5160a9fa2f778890d16d4..458bde016ac2bd19e75dc6a93df1ef72fe0664b6