A separate TEXLIGHTS lump would define which textures give off how much light. For example, the white dots on the ceiling on Doom 2 map 1 would each give off a certain amount of white light, as seen in the screenshot here.
Heck, would it be possible to have a new picture that's a lightmap, with versions of all the textures colored by the amount of light they give off? It'd probably double the size of WADs, but.. for natural light?
