rtc-svc: Reset ref_map_idx for references not used

For layered rtc coding, with the set_ref_frame_config:
for a reference not used reset its ref_map_idx
to the first reference used.

Bug: 362123224

Change-Id: I39a0e57b5d497b60179be4eeaed3cab924a493c4

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 9d66048..93bc505 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -3101,12 +3101,33 @@
         const int gld_ref = get_ref_frame_map_idx(cm, GOLDEN_FRAME);
         aom_wb_write_literal(wb, gld_ref, REF_FRAMES_LOG2);
       }
-
+      int first_ref_map_idx = INVALID_IDX;
+      if (cpi->ppi->rtc_ref.set_ref_frame_config) {
+        for (ref_frame = LAST_FRAME; ref_frame <= ALTREF_FRAME; ++ref_frame) {
+          if (cpi->ppi->rtc_ref.reference[ref_frame - 1] == 1) {
+            first_ref_map_idx = cpi->ppi->rtc_ref.ref_idx[ref_frame - 1];
+            break;
+          }
+        }
+      }
       for (ref_frame = LAST_FRAME; ref_frame <= ALTREF_FRAME; ++ref_frame) {
         assert(get_ref_frame_map_idx(cm, ref_frame) != INVALID_IDX);
-        if (!current_frame->frame_refs_short_signaling)
-          aom_wb_write_literal(wb, get_ref_frame_map_idx(cm, ref_frame),
-                               REF_FRAMES_LOG2);
+        if (!current_frame->frame_refs_short_signaling) {
+          if (cpi->ppi->rtc_ref.set_ref_frame_config &&
+              first_ref_map_idx != INVALID_IDX &&
+              !seq_params->order_hint_info.enable_order_hint) {
+            // For the usage of set_ref_frame_config:
+            // for any reference not used set their ref_map_idx
+            // to the first used reference.
+            const int map_idx = cpi->ppi->rtc_ref.reference[ref_frame - 1]
+                                    ? get_ref_frame_map_idx(cm, ref_frame)
+                                    : first_ref_map_idx;
+            aom_wb_write_literal(wb, map_idx, REF_FRAMES_LOG2);
+          } else {
+            aom_wb_write_literal(wb, get_ref_frame_map_idx(cm, ref_frame),
+                                 REF_FRAMES_LOG2);
+          }
+        }
         if (seq_params->frame_id_numbers_present_flag) {
           int i = get_ref_frame_map_idx(cm, ref_frame);
           int frame_id_len = seq_params->frame_id_length;