Coloc 2: provide the MaskedImage with a LocalizableByDimCursor

This is needed for a correct evaluation of the thresholded Manders
coefficient.

For the moment, that cursor class is mostly a stub; it is just enough to
do the job for Coloc 2.

Initial investigation by Leonardo Guizzetti.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>