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)