Add the debug_threads boolean field to PerThreadContext. For
PerThreadContext *p, p->debug_threads records whether the
FF_DEBUG_THREADS bit is set in p->avctx->debug, and p->debug_threads and
p->avctx->debug are kept in sync. The debug_threads field is defined as
an atomic_int to allow atomic read by another thread in
ff_thread_await_progress().

This fixes the tsan warning that
2e664b9c attempted to fix:

WARNING: ThreadSanitizer: data race (pid=452658)
  Write of size 4 at 0x7b640003f4fc by main thread (mutexes: write M248499):
    #0 update_context_from_user [..]/libavcodec/pthread_frame.c:335:19 (5ab42bb1a6f4b068d7863dabe9b2bacc+0xe73859)
[..]
  Previous read of size 4 at 0x7b640003f4fc by thread T130 (mutexes: write M248502, write M248500):
    #0 ff_thread_await_progress [..]/libavcodec/pthread_frame.c:591:26 (5ab42bb1a6f4b068d7863dabe9b2bacc+0xe749a1)
Signed-off-by: Wan-Teh Chang <wtc@google.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>