The function sc_cond_timedwait() accepted a parameter representing the
max duration to wait, because it internally uses SDL_CondWaitTimeout().
Instead, accept a deadline, to be consistent with
pthread_cond_timedwait().
Commit 21d206f360 added mutex assertions.
However, the "locker" variable to trace the locker thread id was read
and written by several threads without any protection, so it was racy.
Reported by TSAN.
