[BUG] brainmapper stalls between registering the two channels with the `--debug` flag

[adamltyson]

This can be reproduced locally, and only occurs when using brainmapper (not brainreg with two channels), and only when the --debug flag is used.

Raised originally on the image.sc forum: https://forum.image.sc/t/brainmapper-crashing-at-downsample/115448

cc @IgorTatarnikov

[imagesc-bot]

This issue has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/brainmapper-crashing-at-downsample/115448/3