Fix rtcp out quality stats

[atoppi]

We got notified about some spurious values of out link quality metrics.

This PR aims at fixing that unwanted behavior, specifically by:

• parsing the reported total lost as a signed integer since it could be a negative value

  cumulative number of packets lost: 24 bits
    The total number of RTP data packets from source SSRC_n that have
    been lost since the beginning of reception.  This number is
    defined to be the number of packets expected less the number of
    packets actually received, where the number of packets received
    includes any which are late or duplicates.  Thus, packets that
    arrive late are not counted as lost, and the loss may be negative
    if there are duplicates.

• improving the float math when evaluating the metrics
• as a side effect, dealing internally with inbound losses by using signed integer, but still skipping negative cumulative losses in the RTCP reports sent to RTCP peers

  uint32_t expected_interval = ctx->expected - ctx->expected_prior;
  uint32_t received_interval = ctx->received - ctx->received_prior;
  int32_t lost_interval = 0;
  if (expected_interval > received_interval) {
      lost_interval = expected_interval - received_interval;
  }
  ctx->lost += lost_interval;

[lminiero]

Merging :+1: I'll apply the same fixes to 0.x too.