These valuators are not available on all systems. We now check
explicitly for support, instead of returning invalid results.
Additionally, unknown valuator names are now printed to the
debug log.
To do that, we need a custom InputOnly window. This window is only used
for receiving the exit ClientMessage - XI2 input is still received
through the root window.
Fixes issue https://github.com/mono/MonoGame/issues/2711
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
