- Store and output samples as 32-bit floats instead of 16-bit integers.
   This provides 24-25 bits of transparency, allowing for 42-48 dB of
   headroom to do volume control and normalisation without throwing
   away bits or dropping dynamic range below 96 dB CD quality.

 - Perform volume control and normalisation in 64-bit arithmetic.

 - Add a dynamic limiter with configurable threshold, attack time,
   release or decay time, and steepness for the sigmoid transfer
   function. This mimics the native Spotify limiter, offering greater
   dynamic range than the old limiter, that just reduced overall gain
   to prevent clipping.

 - Make the configurable threshold also apply to the old limiter, which
   is still available.

Resolves: librespot-org#608

Usage: `--format {F32|S16}`. Default is F32.

 - Implemented for all backends, except for JACK audio which itself
 only supports 32-bit output at this time. Setting JACK audio to S16
 will panic and instruct the user to set output to F32.

 - The F32 default works fine for Rodio on macOS, but not on Raspian 10
 with Alsa as host. Therefore users on Linux systems are warned to set
 output to S16 in case of garbled sound with Rodio. This seems an issue
 with cpal incorrectly detecting the output stream format.

 - While at it, DRY up lots of code in the backends and by that virtue,
 also enable OggData passthrough on the subprocess backend.

 - I tested Rodio, ALSA, pipe and subprocess quite a bit, and call on
 others to join in and test the other backends.

While at it, add a small tweak when converting "silent" samples
from float to integer. This ensures 0.0 converts to 0 and vice
versa.

 - DRY-ups

 - Remove incorrect optimization attempt in the libvorbis decoder,
   that skewed 0.0 samples non-linear

 - PortAudio and SDL backends do not support S24 output. The PortAudio
   bindings could, but not through this API.

For Rodio, this fixes garbled sound on some but not all Alsa hosts.