Currently, the frame-threaded decoding API still supports thread-unsafe
callbacks. If one uses a thread-unsafe get_buffer2() callback,
calls to av_frame_unref() by the decoder are serialized, because
it is presumed that the underlying deallocator is thread-unsafe.

The frame-threaded encoder seems to have been written with this
restriction in mind: It always serializes unreferencing its AVFrames,
although no documentation forces it to do so.

This commit schedules to change this behaviour as soon as thread-unsafe
callbacks are removed. For this reason, the FF_API_THREAD_SAFE_CALLBACKS
define is reused.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>