The two function were always called together.  This factors out
the NACKing logic into the track.

This avoids carrying an extra parameter in many places.

Now that we have our own samplebuilder, we can use the mark bit
to avoid dropping packets before a dropped packet.

Pion's samplebuilder has been broken for over a month now, switch
to our own.

Fixes the instructions for compling under Windows.

The first time we drop, we may already have mapped a number of
packets with the identity mapping.  Insert an identity mapping
in Drop.

Also extend any existing mapping when inserting out-of-order mappings.

We were not rewriting in this case.

Maintain spatial layer information, and drop lower layers when
possible.  Yields a 20% saving with VP9.

We were requesting two keyframes in a row.

Metadata OBUs can appear between the sequence header and the first
frame header.

RFC 6184 Section 8.5.1 implies that the sender will send a
a sequence parameter set in response to PLI.  Since a keyframe is
useless without parameters, use the SPS to detect keyframes.

Now that we start at 200kbps, we need to increase our rate faster.

Also allow selecting just a single connection.

This avoids issues with initialisation order.