Frame threading -
* Restrictions with slice threading also apply.
-* For best performance, the client should set thread_safe_callbacks if it
- provides a thread-safe get_buffer() callback.
+* Custom get_buffer2() and get_format() callbacks must be thread-safe.
* There is one frame of delay added for every thread beyond the first one.
Clients must be able to handle this; the pkt_dts and pkt_pts fields in
AVFrame will work as usual.