This is a drop-in replacement for Pion's samplebuilder that is faster and
has slightly different behaviour.  It is used by Galene <https://galene.org>.

Differences from Pion:

  * the `Pop` family make slightly different decisions about when to
    release a packet that is blocked by an incomplete packet, with the
    goal to minimise latency in the lossless case while avoiding spurious
    packet drops (it's a tricky decision, and Pion itself behaves
    differently depending on the exact version);

  * the `ForcePopWithTimestamp` function can be used to flush any
    packets remaining at the end of the stream;

  * we're about 30% faster.