[Fix] Fix to non-clustered omni shadow rendering

[mvaligursky]

Sorts out an issue introduced here: #4958

• only calls this.updateLightTextureAtlas for clustered lights

Additionally

• in Lights example adds a missing handler to load cookie cubemap
• does not increment _shadowMapUpdates unless the shadow actually renders
• added error when texture required by the uniform is missing, as that causes hard to debug issues like this:

Now we get a lot more useful engine errors