rtpmux: fix output segment and buffer DTS to correspond to the flattened PTS
rtpmux: fix output segment and buffer DTS to correspond to the flattened PTS

https://bugzilla.gnome.org/show_bug.cgi?id=780347

pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good