Silent Pics MLV timelapse: write the MLVI header for every chunk

Created originally on Bitbucket by dmilligan (David Milligan)

Was already merged in Bitbucket before import, marked as merged by the import user

http://www.magiclantern.fm/forum/index.php?topic=9560.msg125401#msg125401 http://www.magiclantern.fm/forum/index.php?topic=12733.msg125833#msg125833

Not tested, please test, I don't have time.