Fix TkCairo bug when displaying images with partially transparent pixels - #32315

PR summary

For the TkCairo GUI backend, the Cairo image buffer uses premultiplied alpha, and is currently passed directly to Tk for blitting, but Tk (PhotoImage) expects unmultiplied alpha. Thus, partially transparent pixels – which usually requires making the Figure and Axes patches transparent – are not displayed correctly. This bug is only in the GUI; saving the image produces the correct output.

This PR uses an existing cbook function to convert from premultiplied-space alpha to unmultiplied-alpha space, and also adds a fast bailout path to that function when the image is fully opaque.

Example

import matplotlib
matplotlib.use('TkCairo')

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

fig, ax = plt.subplots()
fig.set_facecolor('none')
ax.set_facecolor('none')
ax.set_axis_off()

# Construct a patch that is 50% transparent
rect1 = Rectangle((0, 0), 1, 0.5, facecolor=(1, 0, 0, 0.5))
ax.add_patch(rect1)
ax.text(0.5, 0.25, 'Test', ha='center', va='center')

# Construct an opaque patch with the expected color when the above is composited with a white backdrop
rect2 = Rectangle((0, 0.5), 1, 0.5, facecolor=(1, 0.5, 0.5, 1))
ax.add_patch(rect2)
ax.text(0.5, 0.75, 'Reference', ha='center', va='center')

plt.show()

Before this PR

After this PR

AI Disclosure

No AI was used

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• [N/A] 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