pthread_frame: change the criterium for updating thread contexts

Currently the next thread's context is updated from the previous one's
if the codec descriptor is not marked as intra-only. That is not
entirely correct, since that property does not necessarily imply
anything about how a specific decoder implementation behaves.

Instead, use the presence of the update_thread_context() callback to
decide whether an update should be performed. Fixes races in CFHD,
should cause no behaviour change in any other decoders.

libavcodec/pthread_frame.c

@@ -246,7 +246,7 @@ static int update_context_from_thread(AVCodecContext *dst, AVCodecContext *src,
 {
     int err = 0;
 
-    if (dst != src && (for_user || !(src->codec_descriptor->props & AV_CODEC_PROP_INTRA_ONLY))) {
+    if (dst != src && (for_user || src->codec->update_thread_context)) {
         dst->time_base = src->time_base;
         dst->framerate = src->framerate;
         dst->width     = src->width;