input: let the pointer motion handlers move the pointer

this allows to implement pointer barriers by using a custom handler
diff --git a/src/data-device.c b/src/data-device.c
index 888e606..313278e 100644
--- a/src/data-device.c
+++ b/src/data-device.c
@@ -278,7 +278,8 @@
 }
 
 static void
-drag_grab_motion(struct weston_pointer_grab *grab, uint32_t time)
+drag_grab_motion(struct weston_pointer_grab *grab, uint32_t time,
+		 wl_fixed_t x, wl_fixed_t y)
 {
 	struct weston_drag *drag =
 		container_of(grab, struct weston_drag, grab);
@@ -286,6 +287,8 @@
 	float fx, fy;
 	wl_fixed_t sx, sy;
 
+	weston_pointer_move(pointer, x, y);
+
 	if (drag->icon) {
 		fx = wl_fixed_to_double(pointer->x) + drag->dx;
 		fy = wl_fixed_to_double(pointer->y) + drag->dy;