Fix stuttering with RTSP streams.

Set the rtspsrc buffer-mode to 'slave' which is the default in
gstreamer 1.0 and doesn't cause the stuttering apparent with
the default buffer mode.

Change-Id: I4241fbe638c176ad93f441a3f76a1041ef1cb6bb
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>