OSPRay v1.6.0

• Updated ISPC device to use Embree v3.1+
• Added new ospShutdown API function to aid in correctness and determinism
  of OSPRay API cleanup
• Added Principled and CarPaint materials to the path tracer
• Improved flexibility of the tone mapper
• Improvements to UnstructuredVolume
  • Implemented support for wedges (in addition to tets and hexes)
  • Implemented support for implicit isosurface geometry
  • Implemented support for cell-centered data (as an alternative to
    per-vertex data)
  • Added an option to precompute normals, providing a memory/performance
    tradeoff for applications
• Implemented QuadMesh geometry type to handle quads directly
• Implemented the ability to set 'void' cell values in all volume types
  • When nan is present as a volume's cell value the volume sample will be
    ignored by the scivis renderer
• Fixed support for color strides which were not multiples of sizeof(float)
• Added support for RGBA8 color format to Spheres, which can be set by
  specifying the "colorFormat" parameter as OSP_UCHAR4, or passing the
  "colorData" through an OSPData of OSP_UCHAR4.
• Added ability to configure Embree scene flags via OSPModel parameters
• ospFreeFrameBuffer has been deprecated in favor of using ospRelease to
  free frame buffer handles
• Fixed memory leak caused by incorrect parameter reference counts in ISPC
  device
• Fixed occasional crashes in the MPIOffload device on shutdown
• Various improvements to sample apps and ospray_sg
  • Added new generator nodes, allowing the ability to inject
    programmatically generated scene data (only C++ for now)
  • Bugfixes and improvements to enhance stability and usability