Support files with multiple mdat atoms.

[bradleysepos]

qtfaststart can mistakenly handle files with multiple mdat atoms as already set up when the moov atom is actually at the end of the file. This patch makes certain the moov atom is placed before all mdat atoms.

[bradleysepos]

Found some more edge cases, will reopen when I have solved them.

[bradleysepos]

Good to go now.