rtspsrc: avoid seek DISCONT when only rate changes in same direction
rtspsrc: avoid seek DISCONT when only rate changes in same direction

Not setting DISCONT avoids a noticable delay when seeking
with only rate changing, in the same direction as current
rate.

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