This commit addresses issues #2226 and MusicPlayerDaemon/ncmpc#95.
MPD and sndio volume ranges being different, the formula to transform a
value from one set to the other is in the form of `(a * x + b) / c`
where:
- a = output set max value
- b = adjustment term
- c = input set max value
Previous calculation formula had `b = 0`, scaling values too low and
rendering increment impossible. Having `b = out_maxval / 2` balances the
transformation, ensuring a better spread of values across the output
range.
Closes#2226
dac2c4df9b