Port EDuke32's 8-bit classic-accurate OpenGL shading, visibility, and fog.

Hello,

I ported this code to Megaton and Redux and I figured you may like to have it upstream. It allows Polymost to look more accurate to the classic renderer.

"usenewshading" is improved fog/visibility code.
"usetileshades" generates separate textures in RAM for different shade values of 8-bit ART using the lookup tables. I ported it to your texture manager; I hope I tread lightly.

-H