Do not close provided file handles with libtiff

[radarhere]

Resolves #7042

libtiff closes file handles, but Pillow might still want access afterwards to seek to other frames in the file. To resolve this, TiffImagePlugin duplicates the file handle.

Pillow/src/PIL/TiffImagePlugin.py

Lines 1256 to 1258 in 9a560c7

	# libtiff closes the file descriptor, so pass in a dup.
	try:
	fp = hasattr(self.fp, "fileno") and os.dup(self.fp.fileno())

#5936 had a problem with too many open files, so the following code was added to ensure that if libtiff did not close the file, Pillow would still close the file.

Pillow/src/PIL/TiffImagePlugin.py

Lines 1308 to 1312 in 9a560c7

	if fp:
	try:
	os.close(fp)
	except OSError:
	pass

#7042 doesn't like this behaviour, feeling that it is not thread-safe to close a file handle more than once.

Investigating how exactly libtiff closes file handles, I found that it is _tiffCloseProc that closes the file handle. This is called by TIFFClose, which we call from

Pillow/src/libImaging/TiffDecode.c

Line 723 in 9a560c7

TIFFClose(tiff);

Looking at the code for TIFFClose, something becomes apparent - if we didn't want to call _tiffCloseProc when a file handle is in use, then we could just call TIFFCleanup instead of TIFFClose. Then we don't need to duplicate the file handle and don't need to try and close the duplicated file handle. Testing, this shouldn't break #5936, and passes #7042's reproduction (although that is not necessarily indicative).

[Yay295]

I think a comment here explaining why we're checking this would be good.

[radarhere]

Ok, I've added some comments.