Tempo and pitch on Windows/Android

Some platforms/systems use a sampleRate of 48000. This results in faster, higher pitched play back. The fix is to use the `sampleRate` from the audio context:

```javascript
_loadSong(midiBuf) {
	const optsPtr = this._lib._mid_alloc_options(
		this._audioContext.sampleRate || DEFAULT_SAMPLE_RATE,
		AUDIO_FORMAT,
		NUM_CHANNELS,
		BUFFER_SIZE
	)
```
