gst/gstsegment.c: Set the last_stop to a more meaningful position when configuring the segment. ie. the start/stop of...

Original commit message from CVS:
* gst/gstsegment.c: (gst_segment_set_newsegment_full):
Set the last_stop to a more meaningful position when configuring the
segment. ie. the start/stop of the segment or clipped against the
updated segment boundaries.
* tests/check/gst/gstsegment.c: (GST_START_TEST):
Add some unit tests for the last_stop.