movenc: Support position independent fragments with interleaving

When starting a new track fragment, the tfhd can either continue
from the end of the previous track run (being position independent)
or include an absolute file position (making the fragment
position dependent).

Therefore, we have to write traf/tfhd/trun atoms for all samples
in the interleaved order, which is quite a bit more overhead than
just writing a separate trun atom for each run of consecutive
samples (as we can do if the omit_tfhd_offset flag).