Support pannerAttr for Howler sounds + improve performance

[DigiEggz]

Exposes the pannerAttr field, which allows panner values to be set. By setting the panningModel to "equalpower", which results in crisp sound within Howler, resolving the problem mentioned in https://github.com/goldfire/howler.js/issues/112.

The processing performance of "equalpower" is notable compared to HRTF: https://padenot.github.io/web-audio-perf/#pannernode-when-panningmodel--hrtf

In the future, panner attributes can be passed within the HTML5AudioSource play() function, or anywhere after the sound is active. It could also make sense to set a global pannerAttr that is referenced within the init() function.

public function play():Void
{
    …
    id = parent.buffer.__srcHowl.play();

    untyped parent.buffer.__srcHowl._volume = cacheVolume;
    parent.buffer.__srcHowl.pannerAttr({coneInnerAngle: 50, panningModel: "HRTF”});) // An example of passing two new panner attributes
    // setGain (parent.gain);
    …
}