rtsp-media: Improve skipping trickmode seek.

We can also skip the seek if the end range is already
correct.

Avoids initial seek on play start if playing full stream.

Part-of: <gstreamer/gst-rtsp-server!198>
28 jobs for !198 with backport-mr-194-into-1.18-skip_trickmode_seek in 29 minutes and 32 seconds (queued for 4 seconds)
merge request