Only draw each polygon pixel once

[radarhere]

Helps #4277

When Pillow draws an ellipse, it considers it to be a 360-sided shape. Expressing that within an ordinary width and height means that those edges overlap.

In such a situation, the polygon_generic function will draw pixels more than once. This is usually unnoticed, since a solid pixel on top of another solid pixel is still just a solid pixel. However, if the pixels drawn have a partial opacity, then repeated drawing of a pixel changes the final opacity.

from PIL import Image, ImageDraw
im = Image.new('RGB', (45, 45), (255, 255, 255))
d = ImageDraw.Draw(im, 'RGBA')
d.ellipse([(0, 0), (45, 45)], (0, 0, 255, 100))
im.save('out.png')

As background, the polygon_generic function uses a scanline algorithm, drawing each row in turn,. Within each row, it starts drawing a line at the edge of a polygon, stops at the next edge, starts at the following edge, and so on.

This PR updates the function to only draw each pixel once.

[hugovk]

Thanks!