rtp: send all RTCP packets as compound packets

[maximilianfridrich]

According to RFC 3550 section 6.1 all RTCP packets MUST be sent as compound packets:

   Thus, all RTCP packets MUST be sent in a compound packet of at least
   two individual packets, with the following format:
   ...
   SR or RR:  The first RTCP packet in the compound packet MUST
      always be a report packet to facilitate header validation as
      described in Appendix A.2.  This is true even if no data has been
      sent or received, in which case an empty RR MUST be sent, and even
      if the only other RTCP packet in the compound packet is a BYE.
   ...
   SDES:  An SDES packet containing a CNAME item MUST be included
      in each compound RTCP packet, except as noted in Section 9.1.

With this change, RTCP packets sent with rtcp_quick_send are now sent as compound packets and thereby RFC compliant.

[maximilianfridrich]

PS: Some UAs enforce this strictly (e.g. Asterisk) by implementing the algorithm provided by RFC 3550 Appendix A:

      u_int32 len;        /* length of compound RTCP packet in words */
      rtcp_t *r;          /* RTCP header */
      rtcp_t *end;        /* end of compound RTCP packet */

      if ((*(u_int16 *)r & RTCP_VALID_MASK) != RTCP_VALID_VALUE) {
          /* something wrong with packet format */
      }
      end = (rtcp_t *)((u_int32 *)r + len);

      do r = (rtcp_t *)((u_int32 *)r + r->common.length + 1);
      while (r < end && r->common.version == 2);

      if (r != end) {
          /* something wrong with packet format */
      }