SDP are now provided *before* the pipeline is fully complete. In order to know whether a media is seekable or not therefore requires asking the invididual streams. API: gst_rtsp_stream_seekable https://bugzilla.gnome.org/show_bug.cgi?id=790674
https://bugzilla.gnome.org/show_bug.cgi?id=774416
https://bugzilla.gnome.org/show_bug.cgi?id=774640