rtspsrc: Ignore sendonly/recvonly attributes unless a backchannel is configured

This works around a bug in various ONVIF cameras that implement the
attributes the wrong way around. They still won't work with a
backchannel but at least normal playback will work for the time being.
It restores pre-1.14 behaviour where we would fail to preroll on any SDP
that lists a recvonly stream. For 1.16 a better solution should be

The problem here is that the ONVIF spec has the meaning of the two
attributes the wrong way around in the examples, compared to RFC4566.

diff --git a/gst/rtsp/gstrtspsrc.c b/gst/rtsp/gstrtspsrc.c
index 7143d78..ce17a0e 100644
--- a/gst/rtsp/gstrtspsrc.c
+++ b/gst/rtsp/gstrtspsrc.c
@@ -1774,7 +1774,7 @@
   if (gst_sdp_media_get_attribute_val (media, "recvonly") != NULL &&
       /* We want to setup caps for streams configured as backchannel */
-      !stream->is_backchannel)
+      !stream->is_backchannel && src->backchannel != BACKCHANNEL_NONE)
     goto recvonly_media;
   /* Parse global SDP attributes once */