Attempt memory mapping when tile args is a string

[radarhere]

Here is the logic used to determine whether or not to try memory mapping.

Pillow/src/PIL/ImageFile.py

Lines 187 to 195 in 9694cfc

	if use_mmap:
	# try memory mapping
	decoder_name, extents, offset, args = self.tile[0]
	if (
	decoder_name == "raw"
	and len(args) >= 3
	and args[0] == self.mode
	and args[0] in Image._MAPMODES
	):

It is expecting args to be a tuple, as in the following.

Pillow/src/PIL/SunImagePlugin.py

Line 125 in 9694cfc

self.tile = [("raw", (0, 0) + self.size, offset, (rawmode, stride))]

However, here are a few place where the args for the "raw" tile is just a string.

Pillow/src/PIL/DdsImagePlugin.py

Line 163 in 9694cfc

self.tile = [("raw", (0, 0) + self.size, 0, "L")]

Pillow/src/PIL/PsdImagePlugin.py

Lines 265 to 268 in 9694cfc

	layer = mode[channel]
	if mode == "CMYK":
	layer += ";I"
	tile.append(("raw", bbox, offset, layer))

Pillow/src/PIL/WebPImagePlugin.py

Line 55 in 9694cfc

self.tile = [("raw", (0, 0) + self.size, 0, self.mode)]

Pillow/src/PIL/WebPImagePlugin.py

Line 167 in 9694cfc

self.tile = [("raw", (0, 0) + self.size, 0, self.rawmode)]

So this PR updates the logic in ImageFile to allow for this possibility as well.

Testing it however, this causes WebPImagePlugin to start failing, so I've added load_seek to stop it attempting memory mapping.