matroska-demux: Send instant-rate-change event if requested in the SEEK event
matroska-demux: Send instant-rate-change event if requested in the SEEK event

Short-circuit instant rate change events by generating
a downstream instant-rate-change event and doing no further
seek processing.

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