[X11] Match win32 wheel coordinate system

OpenTK uses the win32 wheel coordinate system, where
(+h, +v) = (right, up). XI2 uses (+h, +v) = (right, down)
instead, so we need to flip the vertical offset.

Fixes issue #133 and https://github.com/mono/MonoGame/issues/2686
[Mac] Match win32 scrolling coordinates
[Mac] Add note for higher precision pixel scroll
[Mac] Fix Mouse.GetState() h-wheel

External mice will now report horizontal scrolling in the low-level
`Mouse.GetState()` API. The touchpad apparently creates an emulated
mouse device that does not report horizontal wheel scrolling events.

Touchpad support probably requires a proper multitouch implementation.
Merge pull request #134 from thefiddler/scrollfix

[Mac][X11] Match win32 wheel coordinate system