Fix WheelEvent incorrectly flipping sign of deltaY

`MouseWheel` incorrectly inverts `deltaY` to be `positive = up`, This is incorrect, `delta*` is supposed to represent the actual scrolling motion of the viewport, which is `positive = down`. The standard isn't really being overly detailed on the subject, but the current way of flipping Y but not X, is weird and inconsistent regardless.

From the W3 standard: `...the value MUST be the measurement along the y-axis to be scrolled...`

```
deltaX of type double, readonly 

In user agents where the default action of the wheel event is to scroll, the
value MUST be the measurement along the y-axis (in pixels, lines, or pages) to
be scrolled in the case where the event is not cancelled. Otherwise, this is an
implementation-specific measurement (in pixels, lines, or pages) of the movement
of a wheel device around the y-axis.
```

I know this is a breaking change, but it seems really bad to break with the standard (behavior) like this for no reason, and should be easy to fix for anyone affected.

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I also removed a half-truthy note about `IE < 9` not supporting `deltaX`, this is true, but it's also true of FireFox, and likely older versions of other major browsers.