Mouse movements (and thus mouse clicks) are wrong if monitors have different y-offset

[croesch]

As mentioned by @tagliola at #98

It also fails if the second monitor is layed out above the first monitor. I have a dual screen setup where the secondary screen is to the right in portrait, and it sticks out slightly above the primary monitor. The clicks are offset vertically at the distance the second monitor sticks out. If I align the tops of the two screens, it works.