Lightgun support in fullscreen using an invisible cursor

[baarreth]

By making the cursor invisible (instead of disabling it), the lightgun will work perfectly in fullscreen mode. This change makes "fullscreen_window" aka "fakefullscreen", and other lightgun hacks, completely unnecessary. I tested on Xubuntu 20.04 x64 and Mate 20.04 arm64, playing Mad Dog. I don't know if it will work on Windows. It does not affect the game when playing using the mouse.