Modify mousewheel rotation calculation

Rather than just passing the raw value returned from
`GET_WHEEL_DELTA_WPARAM` first divide the value by `WHEEL_DELTA` to
indicate the number of rotations made.

Documentation for [WM_MOUSEWHEEL events][1] states that the data stored
in the `wParam` parameter contains the status of various virtual keys,
but more importantly the distance the mouse wheel is rotated __expressed
in multiples of `WHEEL_DELTA`__. If the game is behave nicely with mice
with wheels of varying resolutions then it should be aware of this
delta.

By passing `GET_WHEEL_DELTA_WPARAM(wParam) / WHEEL_DELTA` directly to
`IHardwareMouse::Event` it is possible to ditch the _explicit_ temporary
variable. Such a variable adds little value to readability because of
it's single use.

   [1]: http://msdn.microsoft.com/en-us/library/windows/desktop/ms645617.aspx