util/box: Calculation error of wlr_box_closest_point in multi-screen mode

In multi-screen mode, when the mouse is on the first screen (default position (100,100)), when the first mask is removed, box is (1920,0). If you directly assign *dest_x = box->x, the mouse position on the second screen will become (1920,100), which is wrong. When x < box->x, x should also be added to box, and y should be the same.