Refactor mel.lib.image.centered_at, split out calc
Refactor mel.lib.image.centered_at, split out calc

Split out the calculation part of mel.lib.image.centered_at(), so that
it can more easily be independently tested.

Test Plan:
$ ./meta/run_tests.sh
.. Run 'mel rotomap-edit' and zoom in some places ..

aevri/mel,aevri/mel