The sample data position is the sum of the data offset and the base data offset.
The base data offset is either specified in the stream, or defaults to the first
byte position in the moof box. (We only support one traf per moof currently, so
the offset does not need to be assigned for later track fragments.) The data
position can optionally be offset by a data position read from the trun.
The auxiliary information offset is calculated in the same way, but using an
offset read from the saio box.
Issue: #837
Issue: #861
6d44ec560e