avcodec encoder: disable multithreading

More work is needed before we can support it
Close #8544

modules/codec/avcodec/encoder.c

@@ -317,6 +317,7 @@ int OpenEncoder( vlc_object_t *p_this )
     p_context = avcodec_alloc_context3(p_codec);
     p_sys->p_context = p_context;
     p_sys->p_context->codec_id = p_sys->p_codec->id;
+    p_context->thread_type = 0;
     p_context->debug = var_InheritInteger( p_enc, "avcodec-debug" );
     p_context->opaque = (void *)p_this;