Test padding bits of first frame

[JeromeMartinez]

Currently --check-padding permits to check the padding bits of all files (performance issue).
The only alternative is to disable the check (current default).
An intermediate method could be to check the first frame only (no performance issue, and catch the use case of all files having padding bits not zero), it does not catch all use cases but a use case of a sponsor is caught without big performance impact.
cc @digitensions.