pad-monitor: Update checks for MISSING_DISCONT

pad-monitor: Update checks for MISSING_DISCONT

* Some SEGMENT might be updates caused by calling gst_pad_set_offset(),
  which will send the same segment but with an updated offset and/or
  based field. For those segments, we don't require a DISCONT on the
  following buffer.
* Ignore differences in flags, they aren't relevant for now to figure
  out whether the segment is an update or not
* Ignore difference in 'position', it's only meant for internal usage
  by elements.
* Changes in the end position (stop in forward playback and start in
  reverse playback) are considering updates

Furthermore, also expect a DISCONT flag on the first buffer following
a STREAM_START.