The old code was trying to save time by only scanning until the first
"mdat" (the encoded data for all the frames) and first "moov" (track
headers etc.) atoms were found.
If a "moof" (for segmented headers in MP4 DASH) atom was found during
that scan, it switched to scanning all top level atoms in order to find
all other "moof" atoms.
However, there are DASH files where the first "moof" atom is located
behind the first "mdat" and "moov" atoms. Reading such a file mkvmerge
aborted its top-level atom scan before encountering the "moof" atom and
therefore never switched to DASH mode. The result was that only a small
portion of the file was read: as much as was given via the tables in the
"moov" atom.
The new code does away with that shortcut and always scans all top level
atoms. It will therefore switch to DASH mode even if the first "moof"
atom is located behind the first "mdat" and "moov" atoms.
Part of the fix for #1867.
