rtsp-stream: avoid deadlock when playing a media where some streams are inactive

Submitted by Branko Subasic

Link to original bug (#794966)

Description

If playing a media with multiple streams, all originating from the same single threaded source, such as a a matroska demuxer, the streaming will be blocked when the inactive parts of the pipeline are filled. This is because there are blocking probes attached to those streams.
We can avoid that by letting the probes drop buffers when playing.