This method is called when a keyframe is reached. This will fire the keyframe event.
If the keyframe has been handled, true is returned. Otherwise, false is returned.
@param {Object} eventDescriptor The event descriptor of the keyframe.
@return {Boolean} True if the keyframe was handled, false if not.
function (eventDescriptor) {
        var me = this;

        // only fire keyframe event (and setup the eventDescriptor) once...
        if (!eventDescriptor.defer) {
            me.makeToken(eventDescriptor, 'done');
            me.fireEvent('keyframe', me, eventDescriptor);
        }

        return eventDescriptor.done;
    }