#1 Add Source_ID to the data packet header

[Riccardo Bernardini]

Currently the identity of a PPETP data packet is determined by the "sequence number" value in the header. This is OK as long as there is only one content source. In a multi-source context (e.g. conference) it is necessary to distinguish the reduced packets obtained from the content of differente sources. This ask for a 32 bit Source_ID value (similar to the SSRC or RTP)