When the cursor is not visible, Mouse.SetPosition requires special treatment on Linux

This partially fixes https://github.com/mono/MonoGame/issues/2283. If the window borders are not visible (Window.IsBorderless=true), then this patch completely fixes the problem. In order to get SetPosition working with windows with borders, it is required that OpenTK is built with this patch: https://github.com/opentk/opentk/pull/85