Refresh light uniforms only when needed

After this change, if 'clean' property is added to an uniform and
evaluates to true, the uniform is not uploaded to the GPU. This is used
to avoid needlessly refreshing lights uniforms when the material changes
but the program does not, saving CPU usage.
I'd rather have "skipUpdate" than "needsUpdate", so that the property evaluates to false by default when it it not set. But I'll change the names.