videoaggregator: fix buffer skipping with pad offsets

videoaggregator: fix buffer skipping with pad offsets

The ->skip_buffer implementation in videoaggregator replicates
the behaviour of the aggregate method to determine whether a
buffer can be skipped
(https://bugzilla.gnome.org/show_bug.cgi?id=781928).

This fixes a typo that made it so the start time of the buffer
was calculated against the output segment, not the segment of
the relevant sinkpad, which caused buffers to be skipped when
for example a sinkpad had received a segment which base had
been modified by a pad offset somewhere along the way.

This simply makes the calculation of the buffer start time
identical to the calculation in aggregate()