refactor: rename sointu.Render as AudioBuffer.Fill

The Render name misleading as it did not do the same thing as normal
Synth.Render, because it disregarded time limits. Conceptually, as
the function modifies the state of the synth, it would be better to
be synth.Fill(audioBuffer), but we cannot define methods on
interfaces; therefore, it is audioBuffer.Fill(synth) now.

audio.go

@@ -3,6 +3,7 @@ package sointu
 import (
 	"bytes"
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"math"
 )
@@ -31,6 +32,19 @@ type (
 	}
 )
 
+// Fill fills the AudioBuffer using a Synth, disregarding all syncs and time
+// limits. Note that this will change the state of the Synth.
+func (buffer AudioBuffer) Fill(synth Synth) error {
+	s, _, err := synth.Render(buffer, math.MaxInt32)
+	if err != nil {
+		return fmt.Errorf("synth.Render failed: %v", err)
+	}
+	if s != len(buffer) {
+		return errors.New("in AudioBuffer.Fill, synth.Render should have filled the whole buffer but did not")
+	}
+	return nil
+}
+
 // Wav converts an AudioBuffer into a valid WAV-file, returned as a []byte
 // array.
 //