avcodec/cbs_av1: keep separate reference frame state for reading and writing

In scearios where a Temporal Unit is written right after reading it using the same
CBS context (av1_metadata, av1_frame_merge, etc), the reference frame state used
by the writer must not be the state that's the result of the reader having already
parsed the current frame in question.

This fixes writing Switch frames, and frames using short ref signaling.
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/cbs_av1.c

@@ -939,6 +939,8 @@ static int cbs_av1_read_unit(CodedBitstreamContext *ctx,
         priv->spatial_id  = 0;
     }
 
+    priv->ref = (AV1ReferenceFrameState *)&priv->read_ref;
+
     switch (obu->header.obu_type) {
     case AV1_OBU_SEQUENCE_HEADER:
         {
@@ -1081,6 +1083,8 @@ static int cbs_av1_write_obu(CodedBitstreamContext *ctx,
     td = NULL;
     start_pos = put_bits_count(pbc);
 
+    priv->ref = (AV1ReferenceFrameState *)&priv->write_ref;
+
     switch (obu->header.obu_type) {
     case AV1_OBU_SEQUENCE_HEADER:
         {

libavcodec/cbs_av1.h

@@ -441,7 +441,9 @@ typedef struct CodedBitstreamAV1Context {
     int tile_cols;
     int tile_rows;
 
-    AV1ReferenceFrameState ref[AV1_NUM_REF_FRAMES];
+    AV1ReferenceFrameState *ref;
+    AV1ReferenceFrameState read_ref[AV1_NUM_REF_FRAMES];
+    AV1ReferenceFrameState write_ref[AV1_NUM_REF_FRAMES];
 } CodedBitstreamAV1Context;