Abstract class extended by lightmappers, for use in [LightmapGI]. This class should be extended by custom lightmapper classes. Lightmappers can then be used with [LightmapGI] to provide fast baked global illumination in 3D. Godot contains a built-in GPU-based lightmapper [LightmapperRD] that uses compute shaders, but custom lightmappers can be implemented by C++ modules.