Add a method to RendererServices that returns a pointer to the TextureSystem used by the RS

When creating the ShadingSystem, if the RS has a TS, then use it rather than creating a new TS, and have a public method that you can use to retrieve it.

This is a consequence of the last patch. Basically, it makes it easier to coordinate between a ShadingSystem & RendererServices that share the same TextureSystem. It's not good enough to let them each have a separate TS, even if those TS's share the same underlying IC, because the two TS's may capture different error messages from different calls, oops. You really do want the very same one in general.