This removes the need for multiple texture files to store the mipmap
chain for a texture. As many mipmaps will be loaded as are present in
the DDS file, and any remaining mipmaps will fall back to the old
behavior.
People who make texture packs usually release them using a specific ID
(for instance SX4E01). Users who have a different version of the game
(like the PAL version SX4P01) then need to rename the custom texture
folder to match. This is a lot simpler than renaming every texture file,
as was required with the old texture format, but it's still something
that users can forget to do. To make that unnecessary, this change makes
it possible to use three-character region-free IDs for custom texture
folders, similarly to how game INIs can use three-character IDs. Once
most people have updated to Dolphin versions that include this change,
those who make texture packs will be able to name them with
three-character IDs, removing the need for users to rename anything.
