WIP: rtpbin: Return NO_PREROLL
It may contain a jitterbuffer which is no-preroll, if there is no other element preventing the preroll (like a live source), the pipeline will never start.
Sadly, this will make pipelines that only use rtpbin for sending no-preroll needlessly, but we don't currently have an API to change make an element no-preroll once it has been set to paused.
This can be reproduced with a piepline like
filesrc ! pcapparse ! rtpbin ! fakesink.