This way the instance can be reused, which allows access to non-deprecated `DataFormat` and `AssetType`.

When loading an asset, construct the URL not by using the default data
type of the type of asset being loaded, but the actual data type of the
skeleton asset passed in. This allows the asset requestor to override
the data format, for example when loading a JPEG-format backdrop.

There are a handful of places where the GUI code could use the storage
module directly; converting those is for a later change.