Add dash parameter for line, polygon, and rectangle drawing

[Krishnachaitanyakc]

Summary

Adds a dash parameter to ImageDraw.line(), ImageDraw.polygon(), and ImageDraw.rectangle() that enables drawing dashed outlines, implemented in the Python layer.

• The dash pattern follows the SVG stroke-dasharray specification: a tuple of ints specifying alternating drawn/blank segment lengths (e.g. (10, 5) draws 10px, skips 5px, repeats)
• Odd-length patterns are automatically doubled per SVG spec
• The dash pattern is continuous across connected line segments and polygon/rectangle edges
• Empty dash tuples raise ValueError
• Fill is drawn normally; only the outline/stroke is dashed
• Fully backward compatible: all existing behavior is preserved when dash is not specified

Example usage

from PIL import Image, ImageDraw

im = Image.new("RGB", (200, 200), "white")
draw = ImageDraw.Draw(im)

# Dashed line
draw.line([(10, 100), (190, 100)], fill="black", width=2, dash=(10, 5))

# Dashed rectangle with fill
draw.rectangle([20, 20, 180, 60], fill="lightyellow", outline="blue", width=2, dash=(15, 5, 5, 5))

# Dashed polygon with fill
draw.polygon([(50, 120), (150, 120), (150, 180), (50, 180)], fill="lightblue", outline="red", width=2, dash=(10, 5))

Closes #9127

Test plan

• Added 10 new tests covering dashed lines, polygons, and rectangles
• Tests cover: basic dash, multi-segment lines, odd-length patterns, empty dash error, polygon with/without fill, rectangle with/without fill
• All 281 existing test_imagedraw.py tests continue to pass (backward compatibility verified)
• Reference images generated and included for pixel-exact assertions
• Documentation updated for all three methods with versionadded:: 12.2.0