hlsenc: Calculate the average and actual maximum bitrate of segments

Previously, the bitrate advertised in the master playlist would only
be based on the nominal values in either AVCodecParameters bit_rate,
or via AVCPBProperties max_bitrate. On top of this, a
fudge factor of 10% is added, to account for container overhead.

Neither of these bitrates may be known, and if the encoder is
running in VBR mode, there is no such value to be known. And
the container overhead may be more or less than the given
constant factor of 10%.

Instead, calculate the maximum bitrate per segment based on
what actually gets output from the muxer, and average bitrate
across all segments.

When muxing of the file finishes, update the master playlist
with these values, exposing both the maximum (which previously
was a guesstimate based on the nominal values) via
EXT-X-STREAM-INF BANDWIDTH, and the average via
EXT-X-STREAM-INF AVERAGE-BANDWIDTH.

This makes it possible to use the hlsenc muxer with VBR
encodes, for VOD style muxing.

Signed-off-by: Martin Storsjö <martin@martin.st>

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