`ClipItem::new_box_shadow()`.

If no blur is present, `DisplayListFlattener::add_box_shadow()` calls
`ClipItem::new_rounded_rect()` in its fast path, which calls
`ensure_no_corner_overlap()`. However, in the slow path, which is used when
blur is present, `ClipItem::new_box_shadow()` is called instead. Prior to this
patch, `ClipItem::new_box_shadow()` neglected to call
`ensure_no_corner_overlap()`. This is why the bug manifested itself only when
blur was present.

Closes #2581.