[css-backgrounds-3][css-borders-4] Only the principal box should get a shadow #9286

https://drafts.csswg.org/css-backgrounds-3/#shadow-layers, https://drafts.csswg.org/css-borders-4/#shadow-layers

If an element has multiple boxes, all of them get drop shadows, but shadows are only drawn where borders would also be drawn; see box-decoration-break.

This is wrong: only the principal box should get the shadow. If that box has multiple fragments, then all of them get shadows, according to box-decoration-break.