We could make UnknownTagData hold on to undecoded, raw input data and
write that back out when serializing. But for now, we don't.
On the flipside, this _does_ write unknown tags that have known types.
We could have a mode where we drop unknown tags with known types.
But for now, we don't have that either.
With this, we can for example reencode
/Library/ColorSync/Profiles/WebSafeColors.icc and icc (and other
tools) can dump the output icc file. The 'ncpi' tag with type 'ncpi'
is dropped while writing it, while the unknown tag 'dscm' with
known type 'mluc' is written to the output. (That file is a v2 file,
so 'desc' has to have type 'desc' instead of type 'mluc' which
it would have in v4 files -- 'dscm' emulates an 'mluc' description
in v2 files.)
It only implements serialization of the 7-bit ASCII string, not yet
serialization of the UCS-2 and Macintosh ScriptCode strings.
With this, matrix-based v2 profiles can be reencoded :^)
With this, common v4 profiles, such as embedded into jpgs by iPhones
(when configured to write jpegs) or Pixel phones, are identical to
the input when reexported :^)
With this, simple v4 matrix profiles using parametric curves, such as
Compact-ICC-Profiles/profiles/sRGB-v4.icc, can be completely serialized
and the serialized file can be read again by `icc` :^)
For now, this can write the profile header.
`icc` refuses to dump its contents since the required copyrightTag,
profileDescriptionTag, and required per-device-class tags are missing,
but it looks ok in a hex editor.
