SkyLightCloudBottomOcclusion

BUsePerSampleAtmosphericLightTransmittance is there on the cloud component and not on the light because otherwise we would need optimisation permutations of the cloud shader. And this for the two atmospheric lights ON or OFF. Keeping it simple for now because this changes the look of the cloud, so it is an art/look decision. Occlude the sky light contribution at the bottom of the cloud layer. This is a fast approximation to sky lighting being occluded by cloud without having to trace rays or sample AO texture. Ignored if the cloud material explicitely sets the ambient occlusion value.