webrtc: fix duplicate payload types with RTX and multiple video codecs

Before this patch, there could be duplicate payload types in offers that
have, within a media section, multiple codecs and RTX enabled:

```
m=video 9 UDP/TLS/RTP/SAVPF 96 97 97 <-- HAS DUPLICATES
a=sendrecv
a=rtpmap:96 VP8/90000
a=rtcp-fb:96 nack
a=rtcp-fb:96 nack pli
a=rtcp-fb:96 ccm fir
a=rtcp-fb:96 transport-cc
a=rtpmap:97 H264/90000
a=rtcp-fb:97 nack
a=rtcp-fb:97 nack pli
a=rtcp-fb:97 ccm fir
a=rtcp-fb:97 transport-cc
a=rtpmap:97 rtx/90000  <--------- PT IS DUPLICATE
a=fmtp:97 apt=96
```

Fix this by populating the media_mapping array with all media formats
rather than only the first one. The added test case reproduces the issue,
which fails without this patch.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8259>

subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c

@@ -3608,6 +3608,7 @@ sdp_media_from_transceiver (GstWebRTCBin * webrtc, GstSDPMedia * media,
   for (i = 0; i < gst_caps_get_size (caps); i++) {
     GstCaps *format = gst_caps_new_empty ();
     GstStructure *s = gst_structure_copy (gst_caps_get_structure (caps, i));
+    gint media_pt;
 
     if (i == 0) {
       gst_structure_foreach_id_str (extmap,
@@ -3631,6 +3632,9 @@ sdp_media_from_transceiver (GstWebRTCBin * webrtc, GstSDPMedia * media,
       return FALSE;
     }
 
+    if (gst_structure_get_int (s, "payload", &media_pt))
+      find_or_create_payload_map_for_media_pt (media_mapping, media_pt);
+
     gst_caps_unref (format);
   }
 
@@ -3643,10 +3647,7 @@ sdp_media_from_transceiver (GstWebRTCBin * webrtc, GstSDPMedia * media,
     guint rtx_target_ssrc = -1;
     gint media_pt;
 
-    if (gst_structure_get_int (s, "payload", &media_pt) &&
+    gst_structure_get_int (s, "payload", &media_pt);
-        webrtc->bundle_policy == GST_WEBRTC_BUNDLE_POLICY_NONE)
-      find_or_create_payload_map_for_media_pt (media_mapping, media_pt);
-
     rtx_target_pt = media_pt;
 
     if (!gst_structure_get_int (s, "clock-rate", &clockrate))

subprojects/gst-plugins-bad/tests/check/elements/webrtcbin.c

@@ -6501,6 +6501,49 @@ GST_START_TEST (test_offer_rollback)
 
 GST_END_TEST;
 
+GST_START_TEST (test_video_rtx_no_duplicate_payloads)
+{
+  struct test_webrtc *t = test_webrtc_new ();
+  GstWebRTCRTPTransceiverDirection direction;
+  GstWebRTCRTPTransceiver *trans;
+  GstCaps *caps;
+
+  VAL_SDP_INIT (no_duplicate_payloads, on_sdp_media_no_duplicate_payloads,
+      NULL, NULL);
+  guint media_format_count[] = { 3 };
+  VAL_SDP_INIT (media_formats, on_sdp_media_count_formats,
+      media_format_count, &no_duplicate_payloads);
+  VAL_SDP_INIT (count, _count_num_sdp_media, GUINT_TO_POINTER (1),
+      &media_formats);
+  const gchar *expected_offer_setup[] = { "actpass", };
+  VAL_SDP_INIT (offer_setup, on_sdp_media_setup, expected_offer_setup, &count);
+  const gchar *expected_offer_direction[] = { "sendrecv", };
+  VAL_SDP_INIT (offer, on_sdp_media_direction, expected_offer_direction,
+      &offer_setup);
+
+  t->on_negotiation_needed = NULL;
+  t->on_ice_candidate = NULL;
+  t->on_pad_added = _pad_added_fakesink;
+
+  /* Setup sendrecv transceiver with VP8 and H264.
+   * The RTX's payload type shouldn't be 96 or 97 since those are already taken. */
+  caps = gst_caps_from_string (VP8_RTP_CAPS (96) ";" H264_RTP_CAPS (97));
+  direction = GST_WEBRTC_RTP_TRANSCEIVER_DIRECTION_SENDRECV;
+  g_signal_emit_by_name (t->webrtc1, "add-transceiver", direction, caps,
+      &trans);
+  gst_caps_unref (caps);
+  fail_unless (trans != NULL);
+  g_object_set (trans, "do-nack", TRUE, NULL);
+  gst_object_unref (trans);
+
+  /* We don't really care about the answer in this test */
+  test_validate_sdp (t, &offer, NULL);
+
+  test_webrtc_free (t);
+}
+
+GST_END_TEST;
+
 static Suite *
 webrtcbin_suite (void)
 {
@@ -6602,6 +6645,7 @@ webrtcbin_suite (void)
           sctpdec);
     }
     tcase_add_test (tc, test_offer_rollback);
+    tcase_add_test (tc, test_video_rtx_no_duplicate_payloads);
   } else {
     GST_WARNING ("Some required elements were not found. "
         "All media tests are disabled. nicesrc %p, nicesink %p, "