Further cleanup of lighting code.

- remove thing color from lighting calculations.
- implement alpha textures and inverse sprites for infrared as texture modes. This still requires some handling for the alpha texture mode for non-shader rendering because there is no way in the fixed pipeline to do it. The inverted texture effect can be done with a texture combiner.
- fixed: ThingColor for sprites was set in the wrong place. It must be in the Process function, not in the lighting calculation.
- added functions for isolated calculation of sprites' dynlight color.

src/gl/renderer/gl_renderer.cpp

@@ -338,6 +338,7 @@ void FGLRenderer::DrawTexture(FTexture *img, DCanvas::DrawParms &parms)
 		else 
 		{
 			// This is an alpha texture
+			gl_RenderState.SetTextureMode(TM_REDTOALPHA);
 			gltex->BindPatch(CM_SHADE, 0);
 		}