We can expand to even more rendering passes to combine a large number of lightmaps.
In this case, we first render just the depth buffer information, to be used by all the subsequent passes.
Then, we render the lightmaps, adding them together using glBlendFunc(GL_ONE, GL_ONE).
Finally, the scene's color information is applied, multiplying it by the accumulated lightmap data, by using glBlendFunc(GL_DST_COLOR, GL_ZERO).
Example: multilightmap.cpp