It was giving misleading results before.  We were dumping images
after we did our opaque composite, but also after we'd broadcast
out the resultant image to image-less nodes.  This meant that
nodes that didn't render anything looked as if they rendered the
entire volume.

git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@7440 18c085ea-50e0-402c-830e-de6fd14e8384