The keyframe detection has been moved into codecs.

We store h.264 in Matroska, since we already have the library.
We currently store the video width and height as 0.

Instead of testing whether a sample is a keyframe, we test at the
packet level, then compare timestamps to identify the keyframe.

The trick of calling the onclose callback wasn't correct.

This fixes issues introduced in commit 6ae79f.

We now inform clients of the status of a group (locked, etc.).  Also
cleans up the handling of administrative messages, which solves the
issue of receiving "user" before "joined".

Without that, we might deadlock if the reader is blocked in read.

Thanks to Ludovic Rateau.

We could call addLocal after the remote was closed, which
would cause the local connection to remain forever.

Thanks to Ludovic Rateau.

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.