This API and implementation comes from the Unreal Engine branch of SDL, which
originally called this "SDL_ConfineCursor".
Some minor cleanup and changes for consistency with the rest of SDL_video, but
there are two major changes:
1. The coordinate system has been changed so that `rect` is _window_ relative
and not _screen_ relative, making it easier to implement without having
global access to the display.
2. The UE version unset all rects when passing `NULL` as a parameter for
`window`, this has been removed as it was an unused feature anyhow.
Currently this is only implemented for X, but can be supported on Wayland and
Windows at minimum too.
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