We may have many windows open, which tries to update/change the
xcursor at various points.
Track which cursor is currently loaded, regardless of which window it
was set by. If someone tries to load that very same xcursor again,
simply skip it.
One example is when we've moused over a window that does *not* have
keyboard focus, and then the user clicks, or by some other mean gives
that window keyboard focus. In many cases it will then try to set the
same cursor again (most of the times, the cursor is the same
regardless of keyboard focus, but not always).
This will trigger e.d. keyboard_leave() and wl_pointer_leave() events,
which ensures there aren't any references to the destroyed window from
the global wayland struct.
Call wl_display_roundtrip() to trigger those events *before* we
destroy the window.
We do however need access to it, so provide a pointer. The difference
is that now we can have a *single* wayland instance, but multiple
terminal instances.
