Fix saving MPO with more than one appended image

[radarhere]

Resolves #8977

With each new frame that is saved to an MPO image, an offset is added

Pillow/src/PIL/MpoImagePlugin.py

Lines 65 to 68 in 086e05f

	offsets.append(fp.tell())
	else:
	im_frame.save(fp, "JPEG")
	offsets.append(fp.tell() - offsets[-1])

leading to a longer mpentries

Pillow/src/PIL/MpoImagePlugin.py

Line 81 in 086e05f

mpentries += struct.pack("<LLLHH", mptype, size, data_offset, 0, 0)

meaning the IFD becomes longer

Pillow/src/PIL/MpoImagePlugin.py

Line 85 in 086e05f

ifd[0xB002] = mpentries

We rewind and replace the placeholder data with the IFD

Pillow/src/PIL/MpoImagePlugin.py

Lines 87 to 89 in 086e05f

	fp.seek(mpf_offset)
	fp.write(b"II\x2a\x00" + o32le(8) + ifd.tobytes(8))
	fp.seek(0, os.SEEK_END)

But the placeholder data only ever has the same number of bytes, regardless of the number of frames.

Pillow/src/PIL/MpoImagePlugin.py

Line 55 in 086e05f

b"\xff\xe2" + struct.pack(">H", 6 + 82) + b"MPF\0" + b" " * 82

This PR increases the length of the placeholder data as the number of frames increase.

[hugovk]

Thanks!