Add tests for very small and large scans

[ns-rse]

#796 introduced a fix for single-pixel wide scans which have a scaling in the range of pico-metres and for very large scans in the range of millimetres. The former example that raised this case was non-square (at 1px x 512px).

We should ensure our test suite covers such scenarios.

It would be useful to have additional information on the errors that arose when doing this. In the absence of a bug report documenting this ensure that tests are made with such sample images before #796 was merged.