OSPRay v2.7.1

• Use Open VKL v1.0.1 to fix sporadic slowdowns when rendering
  structured regular and VDB volumes with the SciVis renderer
• Fix CMake variables and logic
• Fix crash when transferfunction.opacity = 0
• Fix bug in MPI data-parallel rendering that caused rendering to hang
• Workaround dynamic linking issue on Windows in MPI distributed
  rendering
• Correctly initialize renderFrame progress
• Improved performance of data-parallel rendering for scenes with
  a large number of regions
• Expanded camera model support of the data-parallel renderer,
  data-parallel rendering can now use all the camera models supported
  by the SciVis renderer
• Clarify documentation and error messages