jpeg_prog_decode_ac_refine treated ZRL (run=15, size=0) as run=16 and
reused the skip loop, which kept consuming correction bits for non-zero
coefficients after the 16th zero-history coefficient. Those bits belong
after the next Huffman symbol (ITU T.81 G.1.2.3, libjpeg
decode_mcu_AC_refine), so the bitstream desynchronised: many progressive
JPEGs, including plain `cjpeg -progressive` output, failed to load or
decoded with wrong pixels.
Skip `run` zeros while refining non-zeros on the way, write the new value
(0 for ZRL) at the next zero, and stop, matching libjpeg's structure.
Add tests/test_image_decode with two cjpeg-generated fixtures that fail
on the old decoder (wrong pixels for grayscale, NULL for 4:2:0) and are
bit-exact with djpeg after the fix. Wire it into `make test`.
(cherry picked from commit 50ed0a4c73)
Salvatore Sanfilippo
korale77