Calling this function in the inner loop of a time critical rendering function apparently wreaks havoc with the CPU instruction cache - even though the function doesn't do anything substantial.
This needs some more in-depth investigation why just having that call can increase sprite rendering time by a factor of 10 and more if there's many lights in view.
- 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.
