Warps the mouse cursor to the center of the currently focused
window or screen when the mouse cursor is
(a) on a different screen, or
(b) on top of a different window.
This version properly handles warping to windows that have not been
mapped yet (before it resulted in a change of the stack order).
See the discussion in (thanks goes to Bakkeby):
https://github.com/bakkeby/patches/issues/60
