UI: Popover arrows are clipped.
This came out after changes in T65351, rBfe3ce615287a.
The clipping can be corrected this way:
https://developer.blender.org/D9030?id=29434
but I'm not sure if there are any side effects.
So I just slightly increased the value of UI_POPUP_MENU_TOP,
but perhaps there is a smarter solution that only affects this arrows.
