Fix the unintended blending of fill/hatch/stroke when drawing patches - #32343

PR summary

This PR fixes a bug in the Agg/Cairo/PGF backends where applying a non-"normal" blend mode to a patch unintentionally resulted in blending the fill/hatch/stroke with each other using that same blend mode. Instead, the correct behavior – which the PDF/SVG backends already do – is for the fill/hatch/stroke to always be blended with each other using the "normal" blend mode, and then the result is blended with the backdrop using the non-"normal" blend mode. This PR makes the Agg/Cairo/PGF backends match the PDF/SVG backends.

The changes to the Cairo/PGF backends are straightforward because we can use open_blend_group()/close_blend_group() to create an isolated blend group for the fill/hatch/stroke. The blend group is used only if an non-"normal" blend mode is specified and if more than one of fill/hatch/stroke is involved. (If there is only one of fill/hatch/stroke, no unintended blending can happen.) The changes to the Agg backend are a little trickier since the drawing function is at a deeper code layer than those renderer methods, so we have to mimic an isolated blend group by manually swapping a fresh buffer in and out of the C++ renderer.

Since this PR is related to blend modes (#31162), it is not to be backported.

In the plots below, the first row is "normal" blend mode, and the second row is "multiply" blend mode. Note how in the "before this PR" image, the part of the stroke that overlaps the fill is being "multiply" blended instead of "normal" blended, and the star hatching plus fill is a different color even over the white backdrop because the stars are getting "multiply" blended with the fill.

Before this PR

After this PR

Generating code

import matplotlib
#matplotlib.use('TkCairo')

import numpy as np

import matplotlib.pyplot as plt
from matplotlib.patches import Circle

data = np.arange(35).reshape((7, 5)) % 7

fc_list = ['none', 'none', 'none', 'none', 'm', 'm', 'm', 'm']
ec_list = ['none', 'c', 'none', 'c', 'none', 'c', 'none', 'c']
lw_list = [0, 5, 0, 5, 0, 5, 0, 5]
hatch_list = [None, None, '*', '*', None, None, '*', '*']
hatchcolor_list = [None, None, 'y', 'y', None, None, 'y', 'y']

fig, axs = plt.subplots(2, 8, figsize=(8, 2), layout='constrained')

for i, blend_mode in enumerate(['normal', 'multiply']):
    for ax, fc, ec, lw, hatch, hatchcolor in zip(axs[i, :], fc_list, ec_list, lw_list,
                                                 hatch_list, hatchcolor_list):
        ax.imshow(data, cmap='jet')

        circle = Circle((3, 3), 3, fc=fc, ec=ec, lw=lw,
                        hatch=hatch, hatchcolor=hatchcolor, blend_mode=blend_mode)
        ax.add_patch(circle)

        ax.autoscale_view()
        ax.set_axis_off()

plt.show()

AI Disclosure

No AI was used

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• New and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New features and API changes have release notes
• [N/A] Documentation complies with general and docstring guidelines