matroskaenc: implement CueRelativePosition

This is a minimal change to matroskaenc that implements CueRelativePosition in the output.
Most players will probably ignore this additional information, but it is in the
matroska spec, and it'd be nice to be able to make use of it.

Signed-off-by: Bernt Habermeier <bernt@wulfram.com>
Tested-by:  wm4 <nfxjfg@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>