Turn an Adobe CMYK JPEG's ink over, and read the /Decode that turns it back

[tannevaled]

A four-component JPEG written by an Adobe tool stores its ink inverted. libjpeg
turns it back, so poppler, mupdf and pdf.js all show a figure on white paper;
Go's image/jpeg targets JPEG files rather than PDFs and does not, so the page
came out almost solid black. Not slightly wrong — black.

Found by comparing every page of the corpus against poppler at the same size
and looking at the worst disagreement, which was 99.98% of the pixels of one
arXiv figure. 35 of 8 300 files carry a CMYK JPEG, 114 images between them, 95
with the Adobe marker, and 68 of the images decode to near-black.

THE HALF THAT WOULD HAVE BROKEN ELEVEN FORMS

Turning the ink over on its own is measurably wrong. Judged against poppler,
file by file, it repairs 23 and breaks 11 — and the 11 are all DVLA forms:

fig_best_baseline.pdf 0.9990 -> 0.0000 repaired
Vas_ablation_compressed.pdf 0.8002 -> 0.0002 repaired
v890-statutory-off-road-... 0.0083 -> 0.9972 broken
application-for-vehicle-tax-v10 0.0047 -> 0.9993 broken

Their JPEGs are indistinguishable from the others — four components, Adobe
APP14, version 100, transform 2 — and the difference is not in the JPEG at all.
It is in the PDF: those eleven carry /Decode [1 0 1 0 1 0 1 0], which asks for
the samples backwards, and this package ignored /Decode on a DCT image
entirely. Two missing halves that cancelled: eleven forms were drawn correctly
only because both were absent at once, and fixing either alone turns them
black.

So both are here. The ink is turned over, and a wholly inverting /Decode turns
it back.

MEASURED AGAINST POPPLER

the 35 files before: median 0.0211, worst 0.9998, 12 under 1%
after: median 0.0017, worst 0.0218, 32 under 1%, all 35
under 5%

Nothing else moved, and every bucket of the wider corpus is the same or better:

1 633 real forms under 1%: 1 518 -> 1 523 worst 0.1775 -> 0.1775
2 223 arXiv under 1%: 2 104 -> 2 112 worst 0.9998 -> 0.6431

WHAT IS STILL NOT DONE, AND IS SAID RATHER THAN LEFT OUT

/Decode is read here only to answer one question — does it invert every
component — and only for a four-component JPEG. A grey or colour JPEG with a
/Decode array is still drawn as though it had none. No corpus file is written
that way, so there is nothing to measure and nothing has been guessed at.

100% statement coverage, go vet and -race clean, nine cross-compile targets.