[feature request] lock mouse in a rectangle

first of all, great package, fits my use case perfectly.
it'd be awesome to be able to lock the mouse in a box (not sure if this would be outside the scope of this crate).
Say for example I have coordinates, width and height.
I want to lock the mouse in that rectangle.
For example calling a function like `fn lock_mouse(x: i32, y: i32, width: i32, height: i32)` would lock the mouse in that rectangle.
kind of an equivalent to winit's `set_cursor_grab` (see [here](https://docs.rs/winit/0.26.1/winit/window/struct.Window.html#method.set_cursor_grab))
and, of course, a function to unlock it as well.