Avoid erroneously showing the zoom indicator at 16% and 33% zoom levels.

This does cause the border to sometimes be drawn a bit "wider" than it should
be (with a gap between it and the canvas), but it's rare and subtle when it
does happen, so I'm not too concerned.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14527 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd