Ignoring Size for a second, we currently have:
Rect::scale_by
Rect::scaled
Point::scale_by
Point::scaled
In Size, before this patch, we have:
Size::scale_by
Size::scaled_by
This aligns Size to use the same method name as Rect and Point. While
subjectively providing API symmetry, this is mostly to allow using this
method in templated helpers without caring what the exact underlying
type is.