pthread_frame: change the way delay is set

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 1f4cf92cfb.

libavcodec/pthread_frame.c

@@ -310,7 +310,6 @@ static int update_context_from_thread(AVCodecContext *dst, AVCodecContext *src,
     }
 
     if (for_user) {
-        dst->delay       = src->thread_count - 1;
 #if FF_API_CODED_FRAME
 FF_DISABLE_DEPRECATION_WARNINGS
         dst->coded_frame = src->coded_frame;
@@ -790,6 +789,9 @@ int ff_frame_thread_init(AVCodecContext *avctx)
     fctx->async_lock = 1;
     fctx->delaying = 1;
 
+    if (codec->type == AVMEDIA_TYPE_VIDEO)
+        avctx->delay = src->thread_count - 1;
+
     for (i = 0; i < thread_count; i++) {
         AVCodecContext *copy = av_malloc(sizeof(AVCodecContext));
         PerThreadContext *p  = &fctx->threads[i];
@@ -827,6 +829,8 @@ int ff_frame_thread_init(AVCodecContext *avctx)
         copy->internal->thread_ctx = p;
         copy->internal->last_pkt_props = &p->avpkt;
 
+        copy->delay = avctx->delay;
+
         if (codec->priv_data_size) {
             copy->priv_data = av_mallocz(codec->priv_data_size);
             if (!copy->priv_data) {