op1a: update start timecode if output start offset updated
op1a: update start timecode if output start offset updated

The start offset could be changed if a client application delayed
the decision until after the essence data was written.

stuarthicks/bmx,kierank/bmxlib-bmx,stuarthicks/bmx,Limecraft/ebu-bmx,xunchangguo/BMX,kierank/bmxlib-bmx,ebu/ebu-bmx,stuarthicks/bmx,ebu/ebu-bmx,Limecraft/ebu-bmx,kierank/bmxlib-bmx,Limecraft/ebu-bmx,ebu/ebu-bmx,xunchangguo/BMX,xunchangguo/BMX