Fix WheelEvent incorrectly flipping sign of deltaY, normalize deltaMode

`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`.

`deltaMode` is assumed to be 0 (pixels) unless reported by the event, this is a currently a correct assumption as we interpret raw wheel delta as roughly equivalent to scrolling in pixels. *However*, a more accurate interpretation would be to report `wheelDelta / 40` (each tick would be 3 lines) and `deltaMode` 1 (lines), so that the user may choose a more accurate conversion for his intended purpose. Should I go ahead with this?

I removed a half-truthy note about `IE < 9` not supporting `deltaX`, this is true, but it's also true of *many* other browsers.