avtp: Add fragmented packets handling to CVF depayloader

avtp: Add fragmented packets handling to CVF depayloader

This patch adds to the CVF depayloader the capability to regroup H.264
fragmented FU-A packets.

After all packets are regrouped, they are added to the "stash" of H.264
NAL units that will be sent as soon as an AVTP packet with M bit set is
found (usually, the last fragment).

Unrecognized fragments (such as first fragment seen, but with no Start
bit set) are discarded - and any NAL units on the "stash" are sent
downstream, as if a SEQNUM discontinuty happened.