Second pass integration.  This code is a lot simpler, actually
works on my Mac, and (most importantly :) can render in parallel.

Still a bit of work left.  Some minor viewing issues, then 2D
transfer functions, and the shaders need some simple updates.
Unfortunately there's a lot of shaders to do this for, so we might
fix shaders ad hoc until some time is reserved for infrastructure
work in Tuvok.

git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@8020 18c085ea-50e0-402c-830e-de6fd14e8384