Feature Request: Custom task schedulers #514

Use case

I'm integrating Embree into a Rust application using Rayon and I'd like to utilize Rayon's thread pool rather than the internal task scheduler. The internal scheduler spawns new threads for each scene commit which is rather heavy. I cannot use TBB because each additional dependency leads to a binary size increase. Embree has already increased my binary size by 3x, even with most features stripped out.