In strict mode (in recent browsers), modifying deltaX on a WheelEvent
throws an exception:
Uncaught TypeError: Cannot set property deltaX of #<WheelEvent> which has only a getter
In non-strict mode or older browsers, assigning to deltaX is just a
no-op.
https://github.com/jquery/jquery-mousewheel/blob/master/jquery.mousewheel.js
is cited in this function; it uses $.event.fix() to make a mutable
jQuery.Event-flavoured copy of the native event before modifying its
properties. Let's do the same.
Fixes#2751
