[pyplot] Eliminate QuadMesh seams in pcolormesh/hist2d output

[Alek99]

Problem

The XY rasterizer used to stroke the shared edges of adjacent mesh cells. That produced visible grid seams in filled pcolormesh and hist2d output even when Matplotlib renders a continuous field.

This root cause directly affects 11 gallery examples using pcolormesh or hist2d, and can affect any downstream QuadMesh rendering.

Comparison

Matplotlib	Corrected xy.pyplot

Before	Difference

The implementation and regression gates are in PR #413.

Complete upstream Matplotlib example

Source: scales/power_norm.py from the supplied Matplotlib 3.11.1 gallery archive.

"""
========================
Exploring normalizations
========================

Various normalization on a multivariate normal distribution.

"""

import matplotlib.pyplot as plt
import numpy as np
from numpy.random import multivariate_normal

import matplotlib.colors as mcolors

# Fixing random state for reproducibility.
np.random.seed(19680801)

data = np.vstack([
    multivariate_normal([10, 10], [[3, 2], [2, 3]], size=100000),
    multivariate_normal([30, 20], [[3, 1], [1, 3]], size=1000)
])

gammas = [0.8, 0.5, 0.3]

fig, axs = plt.subplots(nrows=2, ncols=2)

axs[0, 0].set_title('Linear normalization')
axs[0, 0].hist2d(data[:, 0], data[:, 1], bins=100)

for ax, gamma in zip(axs.flat[1:], gammas):
    ax.set_title(r'Power law $(\gamma=%1.1f)$' % gamma)
    ax.hist2d(data[:, 0], data[:, 1], bins=100, norm=mcolors.PowerNorm(gamma))

fig.tight_layout()

plt.show()

# %%
#
# .. admonition:: References
#
#    The use of the following functions, methods, classes and modules is shown
#    in this example:
#
#    - `matplotlib.colors`
#    - `matplotlib.colors.PowerNorm`
#    - `matplotlib.axes.Axes.hist2d`
#    - `matplotlib.pyplot.hist2d`

Acceptance

• Adjacent mesh cells are filled without unintended boundary strokes.
• Intentional user-specified edges remain supported.
• Mesh geometry, normalization, colorbar, labels, and limits match the Matplotlib structure.
• The filled-vector visual gate passes without a waiver or Matplotlib-renderer fallback.
• All 11 directly affected gallery examples remain ratcheted green.

[Alek99]

Implemented in #413 and revalidated on exact head 99ee677ad05a506b9a4dd11385c708d411b6f43e.

CI run 30669491110 is green. All eight standard gallery shards pass 472/472, including the 11 directly affected pcolormesh/hist2d cases. The filled-vector visual and semantic gates pass with zero renderer fallbacks or waivers.