Restore original encoderinfo after saving

[radarhere]

Resolves #8936

#8483 added a generic way to set additional options when saving subsequent frames.

from PIL import Image

im = Image.open("Tests/images/hopper.png")
second_im = Image.new("RGB", (128, 128))
second_im.encoderinfo = {"xmp": b"Second frame"}
im.save("out.mpo", save_all=True, append_images=[second_im])

In that PR, I deleted the image's encoderinfo at the end of the save process. The user would more likely expect the image's encoderinfo to still hold the same information.