Add min buffer size.

Buffer size is calculated based on the jackd's period. When the period
gets small, the calculated buffer size does not work well with cmus,
causing dropouts. This commit adds a minimum buffer size of 8192, which
seems to work well.
modify BUFFER_SIZE_MIN to 16384

8192 is drops when running with a powersave governor
fixes buffer overflow and channel position tracking

samples might actually be less than the lag. If that is the case, a
bufferoverflow occurs.

position tracking was also brocken, as it compared the bytes read and
not the current channel position.
Merge pull request #121 from nefthy/master

jack: add min buffer size.
Merge pull request #122 from nefthy/jack-underun-buffer-overflow

jack: fix buffer overflow and channel position tracking