Significant speedup in (pre-Chromium) Edge, especially for high
bitrates such as uncompressed or lightly-compressed frames.

Previously, buffer.byteLength and buffer.length got looked up
multiple times per byte processed in Edge, leading to higher
CPU usage.

Allows for creating an MP4 stream with FLAC contents using
the MP4 box generation functions or AudioSegmentStream.

Use codec string 'fLaC' and specify a 'metadata' field in the
track info with the FLAC METADATA_BLOCK_HEADER fields to go
into the dfLa box.

See https://github.com/xiph/flac/blob/master/doc/isoflac.txt
for details of the MP4 embedding of FLAC audio.

Also allows variable-length audio packets in AudioSegmentStream

FLAC may have variable-length packets, so allow passing a
duration in samples on incoming frames.

This gets FLAC with, say, 960 samples per frame working
consistently in Firefox and Chrome.