made viz panel return a preferred size that makes sense (it preferrs to be
one page size as rendered to the screen: which is 72 dpi, but I should
look into whether or not I can avoid hard coding that).
git-svn-id: https://samskivert.googlecode.com/svn/trunk@260 6335cc39-0255-0410-8fd6-9bcaacd3b74c