Provide AudioFormatNum.SILENCE constant

With the current callback function signatures it'll be both easier to
use than AudioSpec.silence and it's correct in all cases.

Closes issue #934.

src/sdl2/audio.rs

@@ -346,31 +346,59 @@ where Self::Channel: AudioFormatNum + 'static
 /// All format types are returned as native-endian.
 pub trait AudioFormatNum {
     fn audio_format() -> AudioFormat;
+
+    /// The appropriately typed silence value for the audio format used.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// // The AudioFormatNum trait has to be imported for the Channel::SILENCE part to work.
+    /// use sdl2::audio::{AudioCallback, AudioFormatNum};
+    ///
+    /// struct Silence;
+    ///
+    /// impl AudioCallback for Silence {
+    ///     type Channel = u16;
+    ///
+    ///     fn callback(&mut self, out: &mut [u16]) {
+    ///         for dst in out.iter_mut() {
+    ///             *dst = Self::Channel::SILENCE;
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    const SILENCE: Self;
 }
 
 /// `AUDIO_S8`
 impl AudioFormatNum for i8 {
     fn audio_format() -> AudioFormat { AudioFormat::S8 }
+    const SILENCE: i8 = 0;
 }
 /// `AUDIO_U8`
 impl AudioFormatNum for u8 {
     fn audio_format() -> AudioFormat { AudioFormat::U8 }
+    const SILENCE: u8 = 0x80;
 }
 /// `AUDIO_S16`
 impl AudioFormatNum for i16 {
     fn audio_format() -> AudioFormat { AudioFormat::s16_sys() }
+    const SILENCE: i16 = 0;
 }
 /// `AUDIO_U16`
 impl AudioFormatNum for u16 {
     fn audio_format() -> AudioFormat { AudioFormat::u16_sys() }
+    const SILENCE: u16 = 0x8000;
 }
 /// `AUDIO_S32`
 impl AudioFormatNum for i32 {
     fn audio_format() -> AudioFormat { AudioFormat::s32_sys() }
+    const SILENCE: i32 = 0;
 }
 /// `AUDIO_F32`
 impl AudioFormatNum for f32 {
     fn audio_format() -> AudioFormat { AudioFormat::f32_sys() }
+    const SILENCE: f32 = 0.0;
 }
 
 extern "C" fn audio_callback_marshall<CB: AudioCallback>
@@ -473,6 +501,10 @@ pub struct AudioSpec {
     pub freq: i32,
     pub format: AudioFormat,
     pub channels: u8,
+    /// The silence value calculated by SDL2. Note that it's inconvenient to use if your channel
+    /// type is not u8 and [incorrect in case of u16](https://bugzilla.libsdl.org/show_bug.cgi?id=4805).
+    /// You're likely to find [the `AudioFormatNum.SILENCE` associated constant](
+    /// trait.AudioFormatNum.html#associatedconstant.SILENCE) more useful.
     pub silence: u8,
     pub samples: u16,
     pub size: u32