Embree v2.6.1

• Major performance improvements for ray tracing subdivision surfaces, e.g. up to 2x faster for scenes where only the tessellation levels are changing per frame, and up to 3x faster for scenes with lots of crease features.
• Initial support for architectures supporting the new 16-wide AVX512 ISA.
• Implemented intersection filter callback support for subdivision surfaces.
• Added RTC_IGNORE_INVALID_RAYS CMake option which makes the ray intersectors more robust against potential full tree traversal caused by invalid ray inputs (e.g. INF, NAN, etc).