Fix pointer enter event without further motion

If the pointer suddenly enters a window but moves no further, we may only get
a POINTER_EVENT_ENTER but no further POINTER_EVENT_MOTION events, so we also
need to handle this event to properly update the selected item.

The issue can be reproduced by moving the mouse programatically, e.g. on Sway:
    swaymsg seat - cursor set 200 200

lib/menu.c

@@ -1190,7 +1190,7 @@ bm_menu_run_with_pointer(struct bm_menu *menu, struct bm_pointer pointer)
         }
     }
 
-    if (pointer.event_mask & POINTER_EVENT_MOTION) {
+    if (pointer.event_mask & (POINTER_EVENT_ENTER | POINTER_EVENT_MOTION)) {
         menu_point_select(menu, pointer.pos_x, pointer.pos_y, displayed);
     }