Only try to connect discontiguous corners at the end of edges

[radarhere]

Resolves #6290

During drawing, #5980 moves a point to join discontiguous corners to the rest of the shape. However, this has been applied too thoroughly, and in #6290, the attempt to join pixels together has ended up dragging pixels from the left hand side of the polygon all the way to the right, creating a gap instead.

Adding a condition to only try and join discontiguous corners at the end of edges fixes the problem.

[nkinnaird]

Hi there, I ran across the scanline issue fixed in this PR myself. Reverting back to 9.0.0 worked fine for me as a fix since I didn't have an issue with the corner points which was the original motivation for these changes. I did notice however that I was also seeing spurious white "fill" points outside the polygon outline. (The spurious points would also appear in some images that don't contain the scanlines.) I'm not sure if that was also fixed in this PR, but thought I'd mention it just in case. The data points for the following polygon image are below just in case it's worth checking out. Thanks!

points = [(1497, 1510), (1500, 1506), (1505, 1509), (1509, 1512), (1513, 1518), (1516, 1522), (1516, 1527), (1515, 1531), (1512, 1534), (1510, 1536), (1507, 1539), (1504, 1542), (1502, 1542), (1498, 1542), (1496, 1540), (1494, 1537), (1490, 1534), (1488, 1529), (1489, 1523), (1492, 1518), (1494, 1514)]

[radarhere]

Using your points and a different fill and outline, here is Pillow 9.0.0

Pillow 9.1.1

and main

So yes, the points outside the outline should also be fixed by this.