Refactor Texture into Texture2D and TextureCube

[Light7734]

As of now we have an enum bvk::Texture::Type that indicates the type of the texture, however this seems like a code smell. first, the enum is never used anywhere (only assigned in different places). second, enumerations and conditionals that indicate different types should really be refactored out by polymorphism. third, TextureCubes and Texture2Ds have very different use cases, and may additionally have different query functions.