reader: handle non-zero material package start pos
reader: handle non-zero material package start pos

Avid files may have non-zero material package track start position
and this is equivalent to non-zero origin in the file source package

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