Adding support for pooling Point objects in PIXI.InteractionData.prototype.getLocalPosition

In certain use cases, such as dragging, events are fired off in rapid succession.

Creating a new PIXI.Point in each event can cause memory usage to build up and
trigger a nasty garbage collection event.

This commit allows you to pass in a point instead of allocating a new one each
time the method is called.

Sample usage would be something like this:

	var newPosition = new PIXI.Point();
	
	myContainer.mousemove = function(data) {
		data.originalEvent.preventDefault();
		
		this.data.getLocalPosition(this.parent, newPosition);
		
		this.position.x = newPosition.x;
		this.position.y = newPosition.y;
	}