splitmux: Shut down element before downward state change
splitmux: Shut down element before downward state change

Make sure the state change won't hang trying to shut down pads
by making sure the streaming has stopped before chaining up.

froggatt/gst-plugins-good-m,ikonst/gst-plugins-good,chamois94/gst-plugins-good,GrokImageCompression/gst-plugins-good,veo-labs/gst-plugins-good,veo-labs/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,vatavuserban/gst-plugins-good,kittee/gst-plugins-good,sh0/gst-plugins-good,kittee/gst-plugins-good,loshca/gst-plugins-good,chamois94/gst-plugins-good,sh0/gst-plugins-good,ikonst/gst-plugins-good,hizukiayaka/gst-plugins-good,ikonst/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,veo-labs/gst-plugins-good,StreamUtils/gst-plugins-good,froggatt/gst-plugins-good-m,loshca/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,shelsonjava/gst-plugins-good,sebras/gst-plugins-good,stfl/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,sh0/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,BigBrother-International/gst-plugins-good,rawoul/gst-plugins-good,rawoul/gst-plugins-good,GrokImageCompression/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,vatavuserban/gst-plugins-good,froggatt/gst-plugins-good-m,GStreamer/gst-plugins-good,chamois94/gst-plugins-good,sebras/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,shelsonjava/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,BigBrother-International/gst-plugins-good,hizukiayaka/gst-plugins-good,kittee/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,BigBrother-International/gst-plugins-good,StreamUtils/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,GStreamer/gst-plugins-good,froggatt/gst-plugins-good-m,rawoul/gst-plugins-good,pexip/gst-plugins-good,loshca/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,shelsonjava/gst-plugins-good,StreamUtils/gst-plugins-good,vatavuserban/gst-plugins-good,sebras/gst-plugins-good,kittee/gst-plugins-good,rawoul/gst-plugins-good,Kurento/gst-plugins-good,sh0/gst-plugins-good,BigBrother-International/gst-plugins-good,Kurento/gst-plugins-good,loshca/gst-plugins-good,stfl/gst-plugins-good,veo-labs/gst-plugins-good,stfl/gst-plugins-good,shelsonjava/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,chamois94/gst-plugins-good,vatavuserban/gst-plugins-good