compositor: Make pointers visible when an output is unplugged

Previously, if a pointer was inside an output that was unplugged, it
could potentialy end up outside any valid output forever. With this
patch, the pointer is moved to the "closest" output to the pointer.
diff --git a/src/compositor.h b/src/compositor.h
index 901366f..d344e37 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -375,6 +375,8 @@
 void
 weston_pointer_set_default_grab(struct weston_pointer *pointer,
 		const struct weston_pointer_grab_interface *interface);
+void
+weston_pointer_verify(struct weston_pointer *pointer);
 
 struct weston_keyboard *
 weston_keyboard_create(void);