fix bug from issue #79

https://developer.mozilla.org/en-US/docs/Web/API/MouseWheelEvent
The method "wheelDeltaX" returns an int but the above class is only
supported by Chrome/Safari and is deprecated so we must use:
https://developer.mozilla.org/en-US/docs/Web/API/WheelEvent
Which returns all doubles.  The least impact is incurred by simply
rounding, which achieves the same behavior as "wheelDeltaX".