mxf op1a: simplify calc of fixed element size
mxf op1a: simplify calc of fixed element size

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