This allows ending up with a normal, non-fragmented file when
the file is finished, while keeping the file readable if writing
is aborted abruptly at any point. (Normally when writing a
mov/mp4 file, the unfinished file is completely useless unless it
is finished properly.)

This results in a file where the mdat atom contains (and hides)
all the moof atoms that were part of the fragmented file structure
initially.
Signed-off-by: Martin Storsjö <martin@martin.st>