This adds a circular buffer to ffmpeg-mux when writing to a file.
Output from ffmpeg is buffered so that slow disk I/O does not block
ffmpeg writes, as this causes the pipe to become full and OBS stops
sending frames with a misleading "Encoding overloaded!" warning. The
buffer may grow to 256 MB depending on the rate of data coming in and
out, if the buffer is full OBS will start waiting in ffmpeg writes.
A separate I/O thread is responsible for processing the contents of
the buffer and writing them to the output file. It tries to process 1 MB
at a time to minimize small I/O.
Complicating things considerably, some formats in ffmpeg require seeking
on the output, so we can't just treat everything as a stream of bytes.
To handle this, we record offsets of each write and try to buffer as
many contiguous writes as possible. This unfortunately makes the code
quite complicated, but hopefully well commented.
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