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()