An end result of #22674 is that `SkinBlueprintContainer`s are only ever
created by supplying a `SkinComponentsContainer` to them. However,
`SkinBlueprintContainer` still contained remnants of code that suggested
it was designed to handle cases where more than the drawable supplied to
it contained more than one `ISerialisableDrawableContainer`, or even
zero.
The zero path is totally dead right now (because every
`SkinComponentsContainer` is *by necessity* an
`ISerialisableDrawableContainer`), and the more-than-one path is dead
*for now* (and potentially forever?). Therefore, just hard-couple
`SkinBlueprintContainer` to receive a single target container.
As pointed out in review, if the current time is after the end of the
slider, the correct hit object to use for sample retrieval is the object
itself, not any nested object.