Refactor: Reworked GL texture manager interface for preparing de::Textures

The older mechanism emphasized image loading over logical texture
variant creation, resulting in a somewhat awkward interface.

When preparing from a general case de::Texture plus a variant spec,
the existing variant selection mechanic can now be implemented as a
function of this process (users of the interface therefore need not
implement it themselves if the automatic logic is suitable).

Todo: Make optional GL_PrepareTexture()'s creation of a new variant.