mouse_scroll_y direction is backwards on Windows

[elidupree]

Currently, the behavior of mouse_scroll_y is inconsistent (going in opposite directions) between Linux and Windows.

The Enigo docs state "Positive numbers for length lets the mouse wheel scroll down and negative ones up". On my Linux and Windows computers, this is behaving correctly on Linux, but backwards on Windows.

Note that these Windows API docs state "A positive value indicates that the wheel was rotated forward, away from the user; a negative value indicates that the wheel was rotated backward, toward the user." This is opposite from Enigo's choice of direction, so the length argument needs to be negated when we call into the Windows API in in win_impl.rs.