It is a constant known at codec init, so set it in
ff_frame_thread_init(). Also, only set it for video, since the meaning
of this field is not well-defined for audio with frame threading.

Fixes availability of delay in callbacks invoked from the per-thread
contexts after 1f4cf92c.

(cherry picked from commit 6943ab688d0c75dbab3222b5b80457ab72a0615f)