widget: Use natural size for allocation for align

This makes align work better with GtkPicture-like widgets. Before it would take the full natural size (which may be bigger than natural size for the allocation), which resulted in overallocations. With GtkPicture itself overallocations aren't noticable with align = center because it centers the image in snapshot.

cc @otte I'm not sure if I'm doing the right thing here, but it seems to fix the behavior for my GtkPicture-like widget, so it doesn't get overallocated at small sizes with h/valign = center.