w32pthreads: move from lavc to compat/

It will be used in other places than lavc.
Move get_logical_cpus() from lavc/pthread to lavu/cpu.

It will be useful in lavfi, and could conceivably be useful to the user
applications as well.
lavfi: add a slice threading infrastructure

Mostly based on libavcodec's
vf_yadif: support slice threading
vf_fade: support slice threading