vtk's opengl extension manager (base class for osmesa
extension manager) had changed in vtk 6.1. we were
overriding all the base class methods using a copy of
an out of date version of vtk class code. as a result
base class was not getting initialized and vtk could
not load any extensions.

I got rid of the overrides that didn't change vtk behavior
and instead use the base class implementation in those
cases.

I also let visit enable the belnd_function_separate
extension. This is used in transparent rendering in
vtk and is required for depth peeling and to get
correct alpha values during readback.


git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@27360 18c085ea-50e0-402c-830e-de6fd14e8384