MPEG-4: Possible 4-byte null terminator (`0x00 00 00 00`) after `udta` in `moov` referenced but not supported

[archaephyrryx]

A long-standing FIXME comment in the mpeg4 implementation references an 'optional 32-bit zero value' that could appear after a stream of udta boxes within a moov box.

However, no actual support is rendered for this case, and it isn't quite clear what format specification this comment is referencing.

The first step should be to find a concrete and authoritative source to explain where exactly such a 4-byte zero value could appear, and then to adapt the mpeg4 format implementation to accommodate these terminators wherever the specification suggests they may appear.