The "progress2" API in pthread_slice.c currently associates a progress
value with a thread rather than a job, relying on the broken assumption
that a job's thread number is equal to its job number modulo thread
count.
This removes this API entirely, and changes hevcdec to use a
ThreadProgress-based implementation that associates a
mutex/cond/progress value with every job.
Fixes races and deadlocks in hevdec with slice threading, e.g. some of
those mentioned in #11221.