Add `IMB_ispic_type_from_memory` so we can detect the file format
of in-memory images.
This removes `is_a_filepath` callback and uses a magic check for
photo-shop files that's compatible with OIIO.
Even though OIIO doesn't support packed images, we can still use the
file magic for detecting the format.
This change allows D9500 (a fix for unpacking images),
to be implemented without a significant performance penalty,
although the actual performance cost would depend heavily on the
blend file.
Reviewed By: dfelinto, sergey
Ref D9517
23c71a5fab