Audio: Introduce _connected flag to fix setFilters bug

[takahirox]

Problem this PR fixes

Audio.setFilters() resets connection by calling .disconnect() and .connect() if .isPlaying is true.

setFilters( value ) {

	if ( ! value ) value = [];

	if ( this.isPlaying === true ) {

		this.disconnect();
		this.filters = value;
		this.connect();

	} else {

		this.filters = value;

	}

	return this;

}

The problem is Audio status can be "connected but .isPlaying is false". In that case connection won't be reset and then filters won't be applied.

The examples of "connected but .isPlaying is false".

• Call .setNodeSource(), setMediaElementSource(), or setMediaStreamSource(). And not call .play().
• Call .play() and then call .pause() or .stop().

How to fix

The root issue would be that .isPlaying is not corresponding to "connected". So I'd like to suggest introducing a new private boolean property ._connected which is updated in .connect/disconnect() and shows exactly whether connected or not. And let setFilters use it to detect whether connected or not.

[mrdoob]

Thanks!