High-resolution volume control and normalisation

- 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/librespot#608

playback/src/audio_backend/subprocess.rs

@@ -48,7 +48,7 @@ impl Sink for SubprocessSink {
         let data: &[u8] = unsafe {
             slice::from_raw_parts(
                 packet.samples().as_ptr() as *const u8,
-                packet.samples().len() * mem::size_of::<i16>(),
+                packet.samples().len() * mem::size_of::<f32>(),
             )
         };
         if let Some(child) = &mut self.child {