ATTR_no_depth deprecation

I am going to take Marginal's suggestion: in the future, ATTR_no_depth will be mapped to ATTR_poly_os 1, and ATTR_depth will be mapped to ATTR_poly_os 0. As far as I can tell, historically the ability to turn off all depth buffering was a misguided attempt to do the kind of things that ATTR_poly_os is meant for.

This implementation will hopefully help any content that is (for some reason) still using ATTR_depth and ATTR_no_depth...modern OBJ generators like the Blender plugin and the AC3D plugin never use this old attribute.