Support writing LONG8 offsets in AppendingTiffWriter

[radarhere]

Pillow/Tests/test_file_tiff.py

Lines 110 to 116 in 3f24276

	with Image.open("Tests/images/hopper_bigtiff.tif") as im:
	# The data type of this file's StripOffsets tag is LONG8,
	# which is not yet supported for offset data when saving multiple frames.
	del im.tag_v2[273]
	outfile = str(tmp_path / "temp.tif")
	im.save(outfile, save_all=True, append_images=[im], tiffinfo=im.tag_v2)

if you try and run the test without removing this tag, an error is raised from AppendingTiffWriter.

    def fixOffsets(
        self, count: int, isShort: bool = False, isLong: bool = False
    ) -> None:
        if not isShort and not isLong:
            msg = "offset is neither short nor long"
>           raise RuntimeError(msg)
E           RuntimeError: offset is neither short nor long

Extending AppendingTiffWriter to handle LONG8 offsets is made complicated as it explicitly only handles shorts and longs, and has duplicated code to achieve that.

Pillow/src/PIL/TiffImagePlugin.py

Lines 2143 to 2151 in 3f24276

	def rewriteLastShort(self, value: int) -> None:
	self.f.seek(-2, os.SEEK_CUR)
	bytes_written = self.f.write(struct.pack(self.shortFmt, value))
	self._verify_bytes_written(bytes_written, 2)
	def rewriteLastLong(self, value: int) -> None:
	self.f.seek(-4, os.SEEK_CUR)
	bytes_written = self.f.write(struct.pack(self.longFmt, value))
	self._verify_bytes_written(bytes_written, 4)

Pillow/src/PIL/TiffImagePlugin.py

Lines 2153 to 2159 in 3f24276

	def writeShort(self, value: int) -> None:
	bytes_written = self.f.write(struct.pack(self.shortFmt, value))
	self._verify_bytes_written(bytes_written, 2)
	def writeLong(self, value: int) -> None:
	bytes_written = self.f.write(struct.pack(self.longFmt, value))
	self._verify_bytes_written(bytes_written, 4)

Pillow/src/PIL/TiffImagePlugin.py

Lines 2198 to 2200 in 3f24276

	def fixOffsets(
	self, count: int, isShort: bool = False, isLong: bool = False
	) -> None:

Rather than just adding in rewriteLastLong8(), readLong8(), and adding an isLong8 parameter, I've reworked the internals of the class to pass around the field size in my first commit. That made adding support for LONG8 very straightforward in my second commit.

[Yay295]

There should probably now also be a check for field_size == 4 and offset >= 2**32.

[radarhere]

It is conceivable that some users might not wish for us to silently upgrade tags to the LONG8 type, since it would be changing the saved image to rely on the BigTIFF specification.