Automatically switch to 64 bit atoms when writing large files

[enzo1982]

This came up in the initial discussion of this fork: #1 (comment)

I.e. there should be an option to make MP4v2 automatically switch to 64 bit atoms (e.g. co64 instead of stco) when writing large files. That way, applications creating MP4 files do not have to figure out upfront whether the output file will have to use 64 bit mode.