Consider this a request for comment. It's the cleanest solution I can
come up with without dropping either the blur, or use of
`ModelBackedDrawable`.
Intended to resolve https://github.com/ppy/osu/issues/14276.
`IHasComboColours` was already mutable (via a strange
`AddComboColours()` method) and exposing a straight list is easier to
work with. `IHasCustomColours` is also similarly externally mutable (in
a way which is not easily removable).