The XNA `Mouse.GetState()` API returns global mouse state, i.e. you still record clicks even when you click outside the GameWindow. (Note that the WinDX platform is broken here.)

WinGL, with and without this PR, correctly matches XNA behavior in `Mouse.GetState()`. What should `Mouse.GetState(GameWindow)` report when the user clicks outside a GameWindow? WinGL records the click, with coordinates relative to the GameWindow argument.