In gst_ogg_demux_do_seek() when calculating the
keyframe time, account for a non-zero start-time

Handle a discontinuous first packet in
gst_ogg_demux_setup_first_granule() because that's pretty
normal after a seek. Also differentiate between a genuinely
truncated first packet and just bailing out early, by not using
granule = -1 as an error code.

Make the debug output logs clearer about which timestamps
are stream times (PTS) and which are ogg timestamps.