feat: add multithreaded rendering to the tracker side

The compiled player does not support multithreading, but with this,
users can already start composing songs with slightly less powerful
machines, even when targeting high-end machines.

Related to #199

audio.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"math"
+	"time"
 )
 
 type (
@@ -62,6 +63,13 @@ type (
 		// Release releases the currently playing note for a given voice. Called
 		// between synth.Renders.
 		Release(voice int)
+
+		// Close disposes the synth, freeing any resources. No other functions should be called after Close.
+		Close()
+
+		// Populates the given array with the current CPU load of each thread,
+		// returning the number of threads / elements populated
+		CPULoad([]CPULoad) int
 	}
 
 	// Synther compiles a given Patch into a Synth, throwing errors if the
@@ -69,7 +77,10 @@ type (
 	Synther interface {
 		Name() string // Name of the synther, e.g. "Go" or "Native"
 		Synth(patch Patch, bpm int) (Synth, error)
+		SupportsMultithreading() bool
 	}
+
+	CPULoad float32
 )
 
 // Play plays the Song by first compiling the patch with the given Synther,
@@ -83,6 +94,7 @@ func Play(synther Synther, song Song, progress func(float32)) (AudioBuffer, erro
 	if err != nil {
 		return nil, fmt.Errorf("sointu.Play failed: %v", err)
 	}
+	defer synth.Close()
 	curVoices := make([]int, len(song.Score.Tracks))
 	for i := range curVoices {
 		curVoices[i] = song.Score.FirstVoiceForTrack(i)
@@ -204,6 +216,17 @@ func (buffer AudioBuffer) Raw(pcm16 bool) ([]byte, error) {
 	return buf.Bytes(), nil
 }
 
+func (p *CPULoad) Update(duration time.Duration, frames int64) {
+	if frames <= 0 {
+		return // no frames rendered, so cannot compute CPU load
+	}
+	realtime := float64(duration) / 1e9
+	songtime := float64(frames) / 44100
+	newload := realtime / songtime
+	alpha := math.Exp(-songtime) // smoothing factor, time constant of 1 second
+	*p = CPULoad(float64(*p)*alpha + newload*(1-alpha))
+}
+
 func (data AudioBuffer) rawToBuffer(pcm16 bool, buf *bytes.Buffer) error {
 	var err error
 	if pcm16 {