Fix putdata() for I;16N on big-endian

[Yay295]

Added some tests for Image methods that deal with bytes, and a test that helper.hopper() works for all image modes.

One of the tests failed though: https://github.com/Yay295/Pillow/actions/runs/5237154342/jobs/9455150887
FAILED Tests/test_image.py::TestImageBytes::test_pixels_after_getdata_putdata[I;16N-1-2] - assert (1, 1029, 515, 1543) == (256, 1284, 770, 1798)

For some reason doing image.putdata(image.getdata()) on an I;16N image on a big-endian machine (s390x) changes the pixels.

[Yay295]

Looks like the problem is here:

Pillow/src/_imaging.c

Lines 1574 to 1590 in 706f8f6

	int endian = strncmp(image->mode, "I;16", 4) == 0 ? (strcmp(image->mode, "I;16B") == 0 ? 2 : 1) : 0;
	double value;
	for (i = x = y = 0; i < n; i++) {
	set_value_to_item(seq, i);
	if (scale != 1.0 || offset != 0.0) {
	value = value * scale + offset;
	}
	if (endian == 0) {
	image->image8[y][x] = (UINT8)CLIP8(value);
	} else {
	image->image8[y][x * 2 + (endian == 2 ? 1 : 0)] = CLIP8((int)value % 256);
	image->image8[y][x * 2 + (endian == 2 ? 0 : 1)] = CLIP8((int)value >> 8);
	}
	if (++x >= (int)image->xsize) {
	x = 0, y++;
	}
	}

The code is handling for endianness for "I;16B", but not "I;16N" on a big-endian machine.

[radarhere]

a test that helper.hopper() works for all image modes

Why would we need to test that this runs without error for all image modes? hopper() isn't part of Pillow, just the test suite. I don't see a need to start testing the tests.

[Yay295]

Yay295 force-pushed the bytes_tests branch

GitHub's suggestion auto-commit added an extra brace for some reason.

Why would we need to test that this runs without error for all image modes? hopper() isn't part of Pillow, just the test suite. I don't see a need to start testing the tests.

You're right. This will essentially be tested just by using it in other tests anyways.

[Yay295]

rebased on main

[Yay295]

Can this be merged?

[hugovk]

This PR has a fix to src/_imaging.c, let's rename the PR about that change, rather than about just adding tests.

[radarhere]

I've created Yay295#13 with some suggestions.

[hugovk]

Thanks!