Do not unnecessarily update FLI __offset

[radarhere]

In FliImagePlugin, self.__offset is initially set to 128, and may then be updated to a different value.

Pillow/src/PIL/FliImagePlugin.py

Lines 76 to 81 in 31eee6e

	self.__offset = 128
	if i16(s, 4) == 0xF100:
	# prefix chunk; ignore it
	self.__offset = self.__offset + i32(s)
	self.fp.seek(self.__offset)

But before it is used again, and before the end of __init__(), it is set back to 128.

Pillow/src/PIL/FliImagePlugin.py

Line 108 in 31eee6e

self.seek(0)

Pillow/src/PIL/FliImagePlugin.py

Lines 128 to 132 in 31eee6e

	def seek(self, frame: int) -> None:
	if not self._seek_check(frame):
	return
	if frame < self.__frame:
	self._seek(0)

Pillow/src/PIL/FliImagePlugin.py

Lines 137 to 143 in 31eee6e

	def _seek(self, frame: int) -> None:
	if isinstance(self._fp, DeferredError):
	raise self._fp.ex
	if frame == 0:
	self.__frame = -1
	self._fp.seek(self.__rewind)
	self.__offset = 128

So it seems simpler to just keep self.__offset at 128.