Bugfix for down scaling already very small images

Fix for https://trac.openmicroscopy.org.uk/ome/ticket/12638 
Ensures that an image can't be down scaled to less than 3px.
Test: Open a very small image (3 or 4px) in full viewer, try to zoom out; nothing should happen if the image would get smaller than 3px (previously: error message was shown).