Default audio buffer length may be too small for some machines about dome HOT 2 CLOSED

I had increased this to 8192 in 7b9e102, but it causes some noticeable latency, which may have caused #10 , so I've reduced it to 4096 again.

Because the audio mixer now runs in a separate thread (see #18), I'm hoping the choppy audio issue is gone, but cross-platform testing will be needed.

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I've reduced this down to 2048 by default, but you can also tune it as a command line parameter by doing ./dome --buffer=n if you find it needs to be resized.

Because the sample buffer has to be a power of 2, n is the power of 2, so to use a size of 2048 (2^11), n = 11.

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