Clear TIFF core image if memory mapping was used for last load

[radarhere]

Resolves #8961

I've used the test image from the issue, but due to a larger file size, I've placed it in python-pillow/test-images#7. This will fail without that pull request.

Using memory mapping to load one frame, and then not using it for the next frame, is causing a problem. This would be because memory mapping is replacing the core image.

Pillow/src/PIL/ImageFile.py

Lines 324 to 326 in 3c71559

	self.im = Image.core.map_buffer(
	self.map, self.size, decoder_name, offset, args
	)

But TiffImagePlugin tries to keep using it afterwards.

Pillow/src/PIL/TiffImagePlugin.py

Lines 1214 to 1223 in 3c71559

	def seek(self, frame: int) -> None:
	"""Select a given frame as current image"""
	if not self._seek_check(frame):
	return
	self._seek(frame)
	if self._im is not None and (
	self.im.size != self._tile_size or self.im.mode != self.mode
	):
	# The core image will no longer be used
	self._im = None

[wiredfool]

Does it make sense to set the c level read-only flag on the image if it's memmapped, and then check if the image is read-only instead? Would the mmap also cause an issue if someone were to try to imagedraw on it?

[radarhere]

I've updated the commit to use the Python readonly flag.

ImageDraw should be fine, thanks to this code in ImageDraw.__init__.

Pillow/src/PIL/ImageDraw.py

Lines 78 to 79 in 3c71559

	if im.readonly:
	im._copy() # make it writeable