perf: optimize sample conversion based on decoder output format

Avoid unnecessary sample conversions in MP3 and WAV decoders by using
cfg attributes to conditionally perform format conversion only when needed.

Author: roderickvd

src/decoder/flac.rs

@@ -123,7 +123,12 @@ where
                 let bits = self.bits_per_sample;
                 let real_val = match bits {
                     8 => (raw_val as i8).to_sample(),
-                    16 => (raw_val as i16).to_sample(),
+                    16 => {
+                        let raw_val = raw_val as i16;
+                        #[cfg(not(feature = "integer-decoder"))] // perf
+                        let raw_val = raw_val.to_sample();
+                        raw_val
+                    }
                     24 => I24::new(raw_val).unwrap_or(Sample::EQUILIBRIUM).to_sample(),
                     32 => raw_val.to_sample(),
                     _ => {

src/decoder/mp3.rs

@@ -110,7 +110,9 @@ where
         let v = self.current_span.data[self.current_span_offset];
         self.current_span_offset += 1;
 
-        Some(v.to_sample())
+        #[cfg(not(feature = "integer-decoder"))] // perf
+        let v = v.to_sample();
+        Some(v)
     }
 }
 

src/decoder/wav.rs

@@ -86,7 +86,13 @@ where
                 (SampleFormat::Float, bits) => {
                     if bits == 32 {
                         let next_f32: Option<Result<f32, _>> = self.reader.samples().next();
-                        next_f32.and_then(|value| value.ok().map(|value| value.to_sample()))
+                        next_f32.and_then(|value| {
+                            value.ok().map(|value| {
+                                #[cfg(feature = "integer-decoder")] // perf
+                                let value = value.to_sample();
+                                value
+                            })
+                        })
                     } else {
                         // > 32 bits we cannot handle, so we'll just return equilibrium
                         // and let the iterator continue
@@ -100,7 +106,13 @@ where
                 }
                 (SampleFormat::Int, 16) => {
                     let next_i16: Option<Result<i16, _>> = self.reader.samples().next();
-                    next_i16.and_then(|value| value.ok().map(|value| value.to_sample()))
+                    next_i16.and_then(|value| {
+                        value.ok().map(|value| {
+                            #[cfg(not(feature = "integer-decoder"))] // perf
+                            let value = value.to_sample();
+                            value
+                        })
+                    })
                 }
                 (SampleFormat::Int, 24) => {
                     let next_i24_in_i32: Option<Result<i32, _>> = self.reader.samples().next();