Overhaul texture management in rendering classes

Overhaul texture management in rendering classes

Textures are now set based on the uniform name (tag) rather than texture
unit.  The Texturing class has been deprecated because it was making
things difficult when textures come from multiple sources, such as with
effects or decals.  RenderPass and Renderer now handle multiple textures
directly.

There's some compatibility code but things are likely to break.