Flat light and back face cull worker mode

Add support for per face lighting and back face culling in worker mode. For larger models, this can offload a lot of calculations to another thread. Typical systems have 4-8 cores / threads to distribute work to.

Worker back face culling is a new property per object. Only works in worker mode (it would probably be a net loss if done on main thread.) This will potentially help some with GPU performance, when lighting is enabled.