gimp_metadata_set_from_exif has special code for handling old-style
GIMP exif parasite data, but didn't check for the more common case
of loading exif data from image formats that can't be handled by exiv2.

The exif data in these formats usually start with the TIFF endian
markers instead of "Exif", which caused a failure to read this
metadata for e.g. EXR images, see issue #10903.

We change this function to check for "Exif" at the start of the data,
in which case we assume it to be the old-style exif parasite and in
that case add extra metadata as was previously always done.

In all other cases, we do not add extra metadata.