rtph264depay: tweak DELTA_UNIT labeling

Consider SPS, PPS and IDR as keyframe, all others as DELTA_UNIT.

See #620154.

gst/rtp/gstrtph264depay.c

@@ -412,14 +412,13 @@ gst_rtp_h264_depay_mark_delta (GstRtpH264Depay * rtph264depay, GstBuffer * nal)
 
   nal_unit_type = (GST_BUFFER_DATA (nal))[4] & 0x1f;
   GST_DEBUG_OBJECT (rtph264depay, "type %d", nal_unit_type);
-  /* non-IDR VCL layer NAL considered DELTA, also AU and SEI;
+  /* SPS/PPS/IDR considered key, all others DELTA;
    * so downstream waiting for keyframe can pick up at SPS/PPS/IDR */
-  if ((nal_unit_type >= 1 && nal_unit_type <= 4) ||
-      (nal_unit_type == 6) || (nal_unit_type == 9)) {
+  if ((nal_unit_type == 5) || (nal_unit_type == 7) || (nal_unit_type == 8)) {
+    GST_BUFFER_FLAG_UNSET (nal, GST_BUFFER_FLAG_DELTA_UNIT);
+  } else {
     GST_BUFFER_FLAG_SET (nal, GST_BUFFER_FLAG_DELTA_UNIT);
     res = TRUE;
-  } else {
-    GST_BUFFER_FLAG_UNSET (nal, GST_BUFFER_FLAG_DELTA_UNIT);
   }
 
   return res;