CMake changed to look for 2.7 or higher and remember major version.
  VTK's macros swallow "VERSION 2.7...<4" signature unfortunately.

Remove deprecated and use newer OSPRay API with 3.x.:
  uint for enums
  box1f instead of vec2f for ranges
  "valueRanges" TransferFunction parameter is now "value".
  linear2d for texture transforms
  no renderertype first argument to new Material
    (ignored since osp 2.7, removed in osp 3.0)
  implicit isocontours take appearance settings from GeometricModel

With OSP 2.x and OptiX use old api where required.