Embree v2.7.1

• Internal tasking system supports cancellation of build operations.
• ISPC mode for robust and compact scenes got significantly faster (implemented hybrid traversal for bvh4.triangle4v and bvh4.triangle4i).
• Hair rendering got faster as we fixed some issues with the SAH heuristic cost factors.
• BVH8 got slight faster for single ray traversal (improves sorting when hitting more than 4 boxes).
• BVH build performance got up to 30% faster on CPUs with high core counts (improved parallel partition code).
• High quality build mode again working properly (spatial splits had been deactivated in v2.7.0 due to some bug).
• Support for merging two adjacent triangles sharing a common edge into a triangle-pair primitive (can reduce memory consumption and BVH build times by up to 50% for mostly quad-based input meshes).
• Internal cleanups (reduced number of traversal kernels by more templating)
• Reduced stack size requirements of BVH builders.
• Fixed crash for dynamic scenes, triggered by deleting all geometries from the scene.