playsinkconvertbin: improve gst_play_sink_convert_bin_getcaps return

If we have the peer caps and a caps filter, return peer_caps +
intersect_first (filter, converter_caps) instead of
intersect_first (filter, peer_caps + converter_caps) and preservers
downstream caps preference order.

https://bugzilla.gnome.org/show_bug.cgi?id=724893
rtspconnection: Add missing include

https://bugzilla.gnome.org/show_bug.cgi?id=725206
rtsp: fix build with older GLib versions

The gio/gnetworking.h header is only available since glib 2.36

https://bugzilla.gnome.org/show_bug.cgi?id=725206