Updated libtiff to 4.2.0

[radarhere]

Resolves #5157

Updated libtiff to 4.2.0, adding a setting to 'winbuild/tiff.opt' because of https://gitlab.com/libtiff/libtiff/-/commit/58b16f47a82323c05ec81f0a821700beb8c2c5a0 (later simplified).

However, from the first commit here, you can see that a test fails.

Investigating, it is failing specifically because of

Pillow/Tests/test_file_libtiff.py

Line 256 in 18ee51a

tc(TiffImagePlugin.IFDRational(4, 7), TiffTags.RATIONAL, True),

libtiff 4.2.0 added the 'Rational2Double' block in this code (if you're interested, it was part of https://gitlab.com/Su_Laus/libtiff/-/commit/6df997c786928757caea0dd68d26ea5f098f49df) -

tv_size = _TIFFDataSize(fip->field_type);
/*--: Rational2Double: For Rationals evaluate "set_field_type" to determine internal storage size. */
if (fip->field_type == TIFF_RATIONAL || fip->field_type == TIFF_SRATIONAL) {
	tv_size = _TIFFSetGetFieldSize(fip->set_field_type);
}
if (tv_size == 0) {
	status = 0;
	TIFFErrorExt(tif->tif_clientdata, module,
		"%s: Bad field type %d for \"%s\"",
		tif->tif_name, fip->field_type,
		fip->field_name);
	goto end;
}

These lines are returning zero for tv_size - I conclude it is because fip->set_field_type is not properly set from https://gitlab.com/libtiff/libtiff/-/blob/v4.2.0/libtiff/tif_dirinfo.c#L1106-1109 in TIFFMergeFieldInfo.

tp->set_field_type =
	 _TIFFSetGetType(info[i].field_type,
		info[i].field_readcount,
		info[i].field_passcount);

This is because _TIFFSetGetType has a few conditions to match against TIFF_RATIONAL -

if (type == TIFF_ASCII && count == TIFF_VARIABLE && passcount == 0)
...
else if (count == 1 && passcount == 0) {
...
else if (count >= 1 && passcount == 0) {
...
else if (count == TIFF_VARIABLE && passcount == 1) {
...
else if (count == TIFF_VARIABLE2 && passcount == 1) {

but none involve count (readcount in TiffDecode.c) being zero.

Pillow/src/libImaging/TiffDecode.c

Line 581 in 18ee51a

int readcount = 0;

So this PR changes that value, and the test now passes.

This also then fixes python-pillow/pillow-wheels#180.