Fix matplotlib >= 3.8 AttributeError in show_factorization_on_image

[Pchambet]

Problem

show_factorization_on_image raises an AttributeError with matplotlib >= 3.8:

AttributeError: 'FigureCanvasAgg' object has no attribute 'tostring_rgb'

FigureCanvasAgg.tostring_rgb() was deprecated in matplotlib 3.8 and removed in 3.10.

Fix

Replace tostring_rgb() with buffer_rgba(), which is the recommended successor:

# Before
data = np.frombuffer(fig.canvas.tostring_rgb(), dtype=np.uint8)
plt.close(fig=fig)
data = data.reshape(fig.canvas.get_width_height()[::-1] + (3,))

# After
data = np.asarray(fig.canvas.buffer_rgba())[..., :3].copy()
plt.close(fig=fig)

Why this works

	tostring_rgb() (old)	buffer_rgba() (new)
Return type	flat bytes	shaped memoryview (H × W × 4)
Channels	RGB (3)	RGBA (4)
Reshape needed	yes (frombuffer + reshape)	no (already H × W × 4)

Since buffer_rgba() returns a properly shaped RGBA array, we:

1. Slice off the alpha channel with [..., :3] to get RGB
2. Call .copy() to detach from the canvas buffer before plt.close() frees it
3. Drop the now-unnecessary reshape call

Backward compatibility

buffer_rgba() has been available since matplotlib 3.1, so this change is backward-compatible with any matplotlib version that this library reasonably supports.

Fixes #556