Makes the currently active rgl object darker by removing a light source. Once all light sources are removed the object can not be any darker.
Examples
x3p <- x3p_read(system.file("sample-land.x3p", package="x3ptools"))
if (FALSE) {
x3p_image(x3p) # run when rgl can open window on the device
x3p_darker() # remove a light source
}