IC/TS: Don't honor SHA-1 fingerprint for files not written by OIIO.

Evil edge case: user creates MIP-mapped texture with maketx (which
writes the hash into either "oiio:SHA-1" metadata, or for formats that
don't support arbitrary metadata, into the "ImageDescription"), then
user alters the image with software like PhotoShop and somehow
re-outputs a MIP-map and figures they don't need to re-maketx it.  Yet
it still contains the metadata with the SHA-1 of the original image, and
so will be recognized by the ImageCache/TextureSystem as an exact
duplicate of the original. Oops.

This patch adds a heuristic wherein if the "Software" metadata does
not start with "OpenImageIO" (which it will, if written by maketx),
the SHA-1 fingerprint will be forgotten.  (In particular, we believe
that PhotoShop will rewrite the "Software" metadata.)  We are also
careful for ImageBufAlgo functions and in oiiotool itself to erase
any fingerprints that are found in the input, before altering images.

This is not perfect -- it can still incorrectly classify an image as a
duplicate if it was altered by a program that does not overwrite the
"Software" metadata, yet alters the pixels.  So don't do that, dummy!
Remove --hash description, no longer used by maketx