PMX importer: Don't use alpha channel of sphere texture.
Follow up to previous commit: Avoid error when failing to load sphere texture.
Avoid error that occurs in Blender built without OpenColorIO.
PMX importer: Set material property "use_cast_shadows" that is available in BLender 2.71 or later.

To keep backward compatibility, "use_cast_buffer_shadows" is still used.
PMX importer: Fix file path issues on Linux and OSX (case-sensitivity and path separators).