pad: add per-pad offsets

pad: add per-pad offsets

When linking pads and when copying a segment event from the sourc pad to the
sinkpad, apply the src and sinkpad offsets to the segment base. Make sure that
we only modify the event stored on the sinkpad and never the one on the source
pad.
When changing the pad offset, perform the segment copy with the updated offsets.
When pushing a segment event, apply the srcpad offset before sending the event
to the peer pad.
This part is missing the adjustment of the segment event on the sinkpad, which
is for a later patch.