jitterbuffer: improve sync on first packets

Don't throw away the first RTCP packet if it arrives before the first
RTP packet but remember and use it to signal sync once we get the
RTP packet.

See https://bugzilla.gnome.org/show_bug.cgi?id=691400

gst/rtpmanager/gstrtpjitterbuffer.c

@@ -275,6 +275,7 @@ gst_rtp_jitter_buffer_clear_pt_map (GstRtpJitterBuffer * jitterbuffer);
 static GstClockTime
 gst_rtp_jitter_buffer_set_active (GstRtpJitterBuffer * jitterbuffer,
     gboolean active, guint64 base_time);
+static void do_handle_sync (GstRtpJitterBuffer * jitterbuffer);
 
 static void
 gst_rtp_jitter_buffer_class_init (GstRtpJitterBufferClass * klass)
@@ -1392,6 +1393,10 @@ gst_rtp_jitter_buffer_chain (GstPad * pad, GstObject * parent,
               priv->clock_rate, &tail, &percent)))
     goto duplicate;
 
+  /* we had an unhandled SR, handle it now */
+  if (priv->last_sr)
+    do_handle_sync (jitterbuffer);
+
   /* signal addition of new buffer when the _loop is waiting. */
   if (priv->waiting && priv->active)
     JBUF_SIGNAL (priv);
@@ -2007,11 +2012,6 @@ do_handle_sync (GstRtpJitterBuffer * jitterbuffer)
 
   priv = jitterbuffer->priv;
 
-  if (priv->last_sr == NULL) {
-    GST_DEBUG_OBJECT (jitterbuffer, "dropping, no SR RTCP");
-    return;
-  }
-
   /* get the last values from the jitterbuffer */
   rtp_jitter_buffer_get_sync (priv->jbuf, &base_rtptime, &base_time,
       &clock_rate, &last_rtptime);
@@ -2065,6 +2065,7 @@ do_handle_sync (GstRtpJitterBuffer * jitterbuffer)
         "sr-buffer", GST_TYPE_BUFFER, priv->last_sr, NULL);
 
     GST_DEBUG_OBJECT (jitterbuffer, "signaling sync");
+    gst_buffer_replace (&priv->last_sr, NULL);
     JBUF_UNLOCK (priv);
     g_signal_emit (jitterbuffer,
         gst_rtp_jitter_buffer_signals[SIGNAL_HANDLE_SYNC], 0, s);