Expose test's #if as command line parameter

If the else part works, this #if should be exposed as a command line parameter and invoked as a new test case.

I finally got around to take a look at the entire code, and this is the only remark I have.

That test might be resurrected with a little work. More generally, we should add a test with 3D images. Also, some examples from DigitalSlideArchive would be useful.