Rebalance dithering, volume ramping and DRC

- Move dithering to actually happen in the ditherAndClamp() procedure to
  reduce memory pressure.

- Make volume ramping occur according to opposite logic: increases of
  volume are slow, decreases rapid. That is a balance that makes far
  more sense and spares user's ears far better.

- Avoid extreme boosts by mapping the dynamic range boost curve on a
  nonlinear function x - x^3/3, for -1 <= x <= 0. This cuts off 1/3rd
  of the entire boosting region and avoids the artifacts arising from
  hardclipping this function.

Signed-off-by: Antti S. Lankila <alankila@gmail.com>