Fix ColorMap length when saving TIFF images with libtiff

[MohammediYunus]

On main, saving a P or PA image through libtiff writes a ColorMap whose green and blue planes are copies of the red plane, shifted by 2 and 4 entries. This is a regression from #10136, which has not been released yet, so I have not added a release note.

from io import BytesIO

from PIL import Image

im = Image.new("P", (1, 1))
im.putpalette((255, 0, 0) * 256)
out = BytesIO()
im.save(out, "TIFF", compression="tiff_lzw")
with Image.open(out) as reloaded:
    print(reloaded.convert("RGB").getpixel((0, 0)))

Pillow 12.3.0 and this PR print (255, 0, 0), but main prints (255, 255, 255). Re-saving an LZW palette TIFF without a compression argument is affected too, since the compression is kept.

#10136 calculates the ColorMap stride from bits_per_sample, which is only set when BitsPerSample has the type TIFF_SHORT.

Pillow/src/encode.c

Lines 987 to 990 in b01a706

	if (type == TIFF_SHORT) {
	if (key_int == TIFFTAG_BITSPERSAMPLE) {
	bits_per_sample = PyLong_AsLong(value);
	}

However, BitsPerSample is a core tag, so its type is not looked up, and since _save() passes it as an int, it is autodetected as TIFF_LONG.

Pillow/src/encode.c

Lines 792 to 796 in b01a706

	if (type == TIFF_NOTYPE) {
	// Autodetect type. Types should not be changed for backwards
	// compatibility.
	if (PyLong_Check(value)) {
	type = TIFF_LONG;

So bits_per_sample stays at 1, and the stride is 2 instead of 256.

Changes proposed in this pull request:

• Read BitsPerSample for the ColorMap stride whenever it is an int, not only when its type is TIFF_SHORT.
• In test_palette_save, compare the RGB content, and also test a PA image. Previously, it only checked that the ColorMap had 768 items.
• In the 16-bit part of test_colormap_size_mismatch, use a non-zero ColorMap and compare its values. Previously, the ColorMap was all zeros and only its length was checked, so the shift was not visible.

Without the src/encode.c change, the P and PA cases of test_palette_save and the libtiff case of test_colormap_size_mismatch fail.

[radarhere]

Thanks very much for catching this before the release!