input: let the pointer motion handlers move the pointer

this allows to implement pointer barriers by using a custom handler
diff --git a/src/compositor.h b/src/compositor.h
index 95928a4..e7ffa4f 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -233,7 +233,8 @@
 struct weston_pointer_grab;
 struct weston_pointer_grab_interface {
 	void (*focus)(struct weston_pointer_grab *grab);
-	void (*motion)(struct weston_pointer_grab *grab, uint32_t time);
+	void (*motion)(struct weston_pointer_grab *grab, uint32_t time,
+		       wl_fixed_t x, wl_fixed_t y);
 	void (*button)(struct weston_pointer_grab *grab,
 		       uint32_t time, uint32_t button, uint32_t state);
 	void (*cancel)(struct weston_pointer_grab *grab);
@@ -359,6 +360,9 @@
 void
 weston_pointer_clamp(struct weston_pointer *pointer,
 			    wl_fixed_t *fx, wl_fixed_t *fy);
+void
+weston_pointer_move(struct weston_pointer *pointer,
+		    wl_fixed_t x, wl_fixed_t y);
 
 struct weston_keyboard *
 weston_keyboard_create(void);