We can't handle recvonly streams, sendonly streams are perfectly fine.

The direction is the one from the point of view of the SDP offerer
(i.e. the RTSP server), and a recvonly stream would be one where the
server expects us to send media.

RFC 3264, section 5.1:
   If the offerer wishes to only send media on a stream to its peer, it
   MUST mark the stream as sendonly with the "a=sendonly" attribute.

This is mixed up in the ONVIF streaming specification examples, but
actual implementations and conformance tools seem to not care at all
about the attributes.

https://bugzilla.gnome.org/show_bug.cgi?id=792376