WIP: rtsp-media: Don't set the sinks to PLAYING on NO_PREROLL
Instead handle the sink states independent of the remainder of the pipeline state. This way we can start producing data in live pipelines while the sinks don't send out data yet until the PLAY/RECORD request is fully handled. This prevents sending data out before the PLAY response and also prevents the PLAY reponse to contain sequence numbers of a packet later than the first one, which then would cause clients to drop the first few packets that were actually received. This allows live medias to work correctly again without sometimes losing the first keyframe, or generally the first packets.
Can be reproduced with
./test-launch "( videotestsrc is-live=true ! timeoverlay ! x264enc tune=zerolatency ! rtph264pay name=pay0 pt=96 audiotestsrc is-live=true ! avenc_aac ! rtpmp4gpay name=pay1 pt=97 )"