Embree v2.6.0

• Added rtcInterpolate function to interpolate per vertex
  attributes
• Added rtcSetBoundaryMode function that can be used to select the
  boundary handling for subdivision surfaces
• Fixed a traversal bug that caused rays with very small ray
  direction components to miss geometry
• Performance improvements for the robust traversal mode
• Fixed deadlock when calling rtcCommit from multiple
  threads on same scene.