In some cases, we'd render earlier than expected, before getting
through RenderSetup.  The cache would be at it's old value then
(except on first renderings!), and wouldn't know to recalculate.
By invalidating here, we'll force this to get recalculated if some
code needs to know about transparencies before RenderSetup
completes.

AFAICT, this also means that `simple' re-renders, such as rotating
the volume, will cause the calculation (and ensuing global
communication) to occur.  This is undesirable, but no worse than a
week ago.

git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@7441 18c085ea-50e0-402c-830e-de6fd14e8384