kmsbasertpendpoint: do-retransmission only for video

Change-Id: I2445595f425326466cef819cd021867da53cdab4
kmsbasertpendpoint: use gst_sdp_media_get_media instead of direct access

Change-Id: I95306d4f0a34a6164eb25c6f554ef14219f085db
kmsbasertpendpoint: set FIR and PLI caps only for video

Change-Id: Ibda6f93e95de6af28bf349285ff17066c93c24d7