Doomsday Engine

Refactor: Textures collection implements a declared state for textures

Textures in the collection can now be declared by name/path prior to
their definition. A defined texture has an associated Texture instance
where as a defined texture exists only as a name/path with an identifier.

Refactor: Textures collection implements name/path bindings

Textures in the collection may now have multiple names which reference
a single Texture instance.

Refactor: Autogenerated Material definitions and sprite rotation sets
are now constructed from declared textures rather than defined ones.

Fixed: Memory leak in the management of PatchComposite definitions.

These refactorings should now allow for deferring the definition of
texture instances until they are first used (not yet implemented).