Add new classes to abstract display setup.
VisItDisplay is an interface for starting up and tearing down rendering contexts. In the case of an XDisplay, it creates an X-based hardware rendering context. A MesaDisplay is a lot of no-ops, with a small bit of code which delegates to InitVTK. git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@5119 18c085ea-50e0-402c-830e-de6fd14e8384