---------- Add access to MTex objects from Lamps and Worlds (first pass). Since the MTex structure is slightly difference between materials, lamps, and worlds, a field is added to the BPy MTex object to distinquish which type it wraps. Attempting to access attributes which are unique to materials for lamp or world MTex objects throw an exception. The next pass will implement MTex attributes which are specific to Lamps and Worlds. A new attribute (textures) is added to each module. It is compatible with the previous Material.getTextures(), which returns a tuple of either MTex objects or None. Surprised we never added an attribute for this before in all the changes and refactoring.