Rearrange ProgramBuilder's standard variables

The rgb_light_foo names were getting a bit stupid, and the hierarchy also
makes more sense this way.  The use of gl_FrontLightProduct was dropped
as too cumbersome to maintain.  The future is with modern shaders anyway.

Texture now only affects ambient and diffuse colors, not specular.  There
will likely be support for a separate specular map in the future.