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.
Improve code style in light uniform optimization
Replace uniform.skipUpdate with uniform.needsUpdate

This is consistent with usage of needsUpdate throughout the rest of the
code. The property is not added to all uniforms for now, so the check
needs to be needsUpdate === false rather than just ! needsUpdate.
Merge pull request #5034 from NVIDIA/optimize-uniform-updates-2

Refresh light uniforms only when needed