Adds the IceT compositing library from Sandia/Kitware.
Most default functionality needed for image compositing is
added, OpenGL acceleration for very large displays is disabled.
Downstream usage:
most VTK based software which supports parallel image compositing,
such as VisIt, Paraview and in our use-case
[ISAAC](https://github.com/ComputationalRadiationPhysics/isaac).
