Values lower than 1.0 can be used to make the fog rendering not fully
obstruct the sky. This can be desired when using fog as a purely
atmospheric effect, without intending to use fog for open world fog
fading.
When set to 0.0, fog rendering behavior will be similar to Godot 3.x
where sky rendering was never affected by fog.
This allows light sources to be specified in physical light units in addition to the regular energy multiplier. In order to avoid loss of precision at high values, brightness values are premultiplied by an exposure normalization value.
In support of Physical Light Units this PR also renames CameraEffects to CameraAttributes.
