avcodec/vvcdec: fix seeking for open GOP

how to reproduce:
wget https://media.xiph.org/video/derf/y4m/students_cif.y4m
vvencapp --input students_cif.y4m --preset faster --output students.266
MP4Box -add students.266:fps=30000/1001:par=12:11 -new students.mp4
ffplay students.mp4

libavcodec/vvc/vvc_refs.c

@@ -80,6 +80,12 @@ void ff_vvc_clear_refs(VVCFrameContext *fc)
             VVC_FRAME_FLAG_SHORT_REF | VVC_FRAME_FLAG_LONG_REF);
 }
 
+void ff_vvc_flush_dpb(VVCFrameContext *fc)
+{
+    for (int i = 0; i < FF_ARRAY_ELEMS(fc->DPB); i++)
+        ff_vvc_unref_frame(fc, &fc->DPB[i], ~0);
+}
+
 static void free_progress(FFRefStructOpaque unused, void *obj)
 {
     FrameProgress *p = (FrameProgress *)obj;

libavcodec/vvc/vvc_refs.h

@@ -33,6 +33,7 @@ int ff_vvc_frame_rpl(VVCContext *s, VVCFrameContext *fc, SliceContext *sc);
 int ff_vvc_slice_rpl(VVCContext *s, VVCFrameContext *fc, SliceContext *sc);
 void ff_vvc_unref_frame(VVCFrameContext *fc, VVCFrame *frame, int flags);
 void ff_vvc_clear_refs(VVCFrameContext *fc);
+void ff_vvc_flush_dpb(VVCFrameContext *fc);
 
 typedef enum VVCProgress {
     VVC_PROGRESS_MV,

libavcodec/vvc/vvcdec.c

@@ -922,9 +922,15 @@ static av_cold void vvc_decode_flush(AVCodecContext *avctx)
 {
     VVCContext *s  = avctx->priv_data;
     int got_output = 0;
+    VVCFrameContext *last;
 
     while (s->nb_delayed)
         wait_delayed_frame(s, NULL, &got_output);
+
+    last = get_frame_context(s, s->fcs, s->nb_frames - 1);
+    ff_vvc_flush_dpb(last);
+
+    s->eos = 1;
 }
 
 static av_cold int vvc_decode_free(AVCodecContext *avctx)