Closes #30934

Summary

This PR adds support for the gapcolor property in Patches, similar to the existing gapcolor feature in Line2D and LineCollection. This allows drawing "stripey" patch edges with two alternating colors in dashed patterns.

Use Case

Drawing unfilled patches on backgrounds of unknown color (very light or very dark), where alternating black/white edges ensure the patch is always visible.

Example

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

fig, ax = plt.subplots()
rect = Rectangle((0.1, 0.1), 0.5, 0.5, fill=False,
                  edgecolor='black', gapcolor='white',
                  linestyle='--', linewidth=3)
ax.add_patch(rect)
plt.show()

Changes

• Add gapcolor parameter to Patch.__init__
• Add get_gapcolor() and set_gapcolor() methods
• Add is_dashed() method to check if edge is dashed
• Modify _draw_paths_with_artist_properties() to draw gaps first with inverse dash pattern when gapcolor is set
• Update update_from() to copy gapcolor
• Add tests for the new functionality

Implementation

Following the same approach as Line2D:

1. If dashed and gapcolor is set, first draw the path with gapcolor using an inverse dash pattern (fills the gaps)
2. Then draw the normal dashed edge on top