OSPRay v2.4.0

• The pathtracer optionally allows for alpha blending even if the
  background is seen through refractive objects like glass, by
  enabling backgroundRefraction
• OSPRay now requires minimum Open VKL v0.11.0 to bring the following
  improvements:
  • Improved rendering performance of VDB volumes
  • Added support for configurable iterator depth via the
    maxIteratorDepth parameters for unstructured and particle
    volumes, improved performance
  • Added support for filter modes for structured volumes (regular
    and spherical)
• Expose parameter horizonExtension of Sun-sky light, which extends
  the sky dome by stretching the horizon over the lower hemisphere
• Optimize handling of geometry lights by the pathtracer
• The optional denoiser image operation now respects frame
  cancellation, requiring Intel® Open Image Denoise with minimum
  version 1.2.3
• Fixed normals of (transformed) isosurfaces
• Robust calculation of normals of boxes geometry
• Clipping geometry is now working correctly with map_maxDepth
  renderer parameter
• Using materials in a renderer with a mismatched renderer_type no
  longer causes crashes while rendering