matroskademux: maintain variable state when searching for position
matroskademux: maintain variable state when searching for position

... so skipping to next cluster happens efficiently

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