Improvements to the engine.

  * Add support for setting the directional attenuation factor. This is
    useful for allowing other effects to control the directional
    attenuation, such as HRTF effects.

  * Add a function for retrieving the index of the listener that a
    sound will be spatialized against based on the current state. A
    function to retrieve the direction to that listener has also been
    added. This is useful for when an effect needs to know information
    about the listener for some kind of spatialization.

  * A bug has been fixed where silence is output if a node has zero
    inputs.

  * A bug has been fixed where a bad channel map can be used for
    spatialization.