BUG: fix fiducials display to preserve screenspace size

This change compensates for the size of the slice view
when displaying fiducial glyphs and text.  Without this change,
switching to a layout showing only a single slice would lead
to humerously large fiducials being displayed.  The base size can
still be controlled via the display nodes.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22091 3bd1e089-480b-0410-8dfb-8563597acbee