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.