matroskademux: don't discard the incoming seek segment on push based seeking
matroskademux: don't discard the incoming seek segment on push based seeking

The incoming seek segment was being discarded leading to push based seeking
being potentially inaccurate.

freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good