avcodec/libdav1d: don't depend on the event flags API to init sequence params the first time

A bug was found in dav1d <= 1.0.0 where the event flag New Sequence Header would
not be signaled for some samples using delayed random access points.
It has since been fixed, but nonetheless it's best to ensure the AVCodecContext
is filled with parameters when parsing the first frame, regardless of what events
were signaled.

Fixes ticket #9694.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/libdav1d.c

@@ -382,7 +382,8 @@ static int libdav1d_receive_frame(AVCodecContext *c, AVFrame *frame)
 
 #if FF_DAV1D_VERSION_AT_LEAST(5,1)
     dav1d_get_event_flags(dav1d->c, &event_flags);
-    if (event_flags & DAV1D_EVENT_FLAG_NEW_SEQUENCE)
+    if (c->pix_fmt == AV_PIX_FMT_NONE ||
+        event_flags & DAV1D_EVENT_FLAG_NEW_SEQUENCE)
 #endif
     libdav1d_init_params(c, p->seq_hdr);
     res = ff_decode_frame_props(c, frame);