Remove image_chunk_dump() and its call-site. I used it for debugging
in the beginning, but not at the end. So, its value is marginal.

Below a performance comparison while running the unit test:

x old
+ new

N           Min           Max        Median           Avg        Stddev

x 5 57.48 58.28 57.58 57.736 0.33886576
+ 5 46.82 47.61 46.93 47.034 0.32791767
Difference at 95.0% confidence
-10.702 +/- 0.486298
-18.5361% +/- 0.842279%
(Student's t, pooled s = 0.333437)