avcodec/mpeg4videodec: update exported AVOptions in the user-facing context

This prevents bogus values being reported on frame multithreaded decoding
scenarios.

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

libavcodec/mpeg4videodec.c

@@ -3495,6 +3495,18 @@ static int mpeg4_update_thread_context(AVCodecContext *dst,
 
     return 0;
 }
+
+static int mpeg4_update_thread_context_for_user(AVCodecContext *dst,
+                                                const AVCodecContext *src)
+{
+    MpegEncContext *m = dst->priv_data;
+    const MpegEncContext *m1 = src->priv_data;
+
+    m->quarter_sample = m1->quarter_sample;
+    m->divx_packed    = m1->divx_packed;
+
+    return 0;
+}
 #endif
 
 static av_cold void mpeg4_init_static(void)
@@ -3585,6 +3597,7 @@ AVCodec ff_mpeg4_decoder = {
     .pix_fmts              = ff_h263_hwaccel_pixfmt_list_420,
     .profiles              = NULL_IF_CONFIG_SMALL(ff_mpeg4_video_profiles),
     .update_thread_context = ONLY_IF_THREADS_ENABLED(mpeg4_update_thread_context),
+    .update_thread_context_for_user = ONLY_IF_THREADS_ENABLED(mpeg4_update_thread_context_for_user),
     .priv_class = &mpeg4_class,
     .hw_configs            = (const AVCodecHWConfigInternal *const []) {
 #if CONFIG_MPEG4_NVDEC_HWACCEL