basesink: Only drop buffer if their PTS is out of segment

basesink: Only drop buffer if their PTS is out of segment

As of now, even for stream completly inside segment, there is no
guarantied that the DTS will be inside the segment. Specifically
for H.264 with B-Frames, the first few frames often have DTS that
are before the segment.

Instead of using the sync timestamp to clip out of segment buffer,
take the duration from the start/stop provided by the sub-class, and
check if the pts and pts_end is out of segment.

https://bugzilla.gnome.org/show_bug.cgi?id=752791