jensenn

This supports boxes needed for reading and writing mp4 files with common encryption. My only concern is that some boxes are getting a lot of children boxes, for example, the...

This has no effective change in release builds, but removes a panic in debug builds.

This add simple reading of uuid boxes at the top level

This writes the mvex box if available

This allows the sample size to be obtained from the mvex box and the tfhd box in addition to the trun sample entry