If pasting an image onto itself at a lower position, copy from bottom

[radarhere]

Resolves #8881

When pasting an image,

Pillow/src/libImaging/Paste.c

Lines 47 to 49 in c8d98d5

	for (y = 0; y < ysize; y++) {
	memcpy(imOut->image[y + dy] + dx, imIn->image[y + sy] + sx, xsize);
	}

will copy the rows of the image from top to bottom.

However, if the image is pasting onto itself, but say, 10 pixels lower,
after those 10 rows, it will start reading from a part of the image that has already been written to. This results in a repeating effect.

from PIL import Image
im = Image.open("Tests/images/hopper.png")
im.paste(im, (0, 10))

A solution for that situation is to instead copy the rows of the image from bottom to top.

[ghost]

It seems there is a different C-code that is called if a mask is available.
Maybe your fix works here, too. Have you tested it with a mask?

mask = Image.new("1", img.size, 1) # mask without an effect just to make sure that also the mask is tested

img.paste(
    img,
    box=(0, 10),
    mask=mask,
)
img.save("img_pasted.png")

[radarhere]

Ok, I've pushed a commit to cover masks as well.

[hugovk]

Can this be a function instead?

[radarhere]

radarhere@be8209e is what would be required. I don't think it's as neat, but let me know if you disagree.

[hugovk]

OK, let's go with this PR version.