bmxtranswrap: fix essence mark locator position adjustment
bmxtranswrap: fix essence mark locator position adjustment

The locator position in the output file equals the input file's
position minus the starting input file's position

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